diff --git a/CMakeLists.txt b/CMakeLists.txt
index ead4696a7..b0adcb598 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -8,6 +8,10 @@ project(dispatch
         LANGUAGES C CXX)
 enable_testing()
 
+if("${CMAKE_C_SIMULATE_ID}" STREQUAL "MSVC")
+  include(ClangClCompileRules)
+endif()
+
 set(CMAKE_C_STANDARD 11)
 set(CMAKE_C_STANDARD_REQUIRED YES)
 
@@ -262,10 +266,6 @@ if (HAVE_DECL_PROGRAM_INVOCATION_SHORT_NAME)
 endif()
 check_symbol_exists(__printflike "bsd/sys/cdefs.h" HAVE_PRINTFLIKE)
 
-if(CMAKE_SYSTEM_NAME STREQUAL Windows)
-  add_definitions(-DTARGET_OS_WIN32)
-endif()
-
 if(CMAKE_SYSTEM_NAME STREQUAL Android)
   set(ENABLE_DTRACE_DEFAULT OFF)
 endif()
@@ -321,6 +321,13 @@ configure_file("${CMAKE_SOURCE_DIR}/cmake/config.h.in"
                "${CMAKE_BINARY_DIR}/config/config_ac.h")
 add_definitions(-DHAVE_CONFIG_H)
 
+if(CMAKE_SYSTEM_NAME STREQUAL Windows)
+  include(DispatchWindowsSupport)
+  dispatch_windows_arch_spelling(${CMAKE_SYSTEM_PROCESSOR} DISPATCH_MSVC_ARCH)
+  dispatch_windows_include_for_arch(${DISPATCH_MSVC_ARCH} DISPATCH_INCLUDES)
+  include_directories(BEFORE SYSTEM ${DISPATCH_INCLUDES})
+endif()
+
 add_subdirectory(dispatch)
 add_subdirectory(man)
 add_subdirectory(os)
diff --git a/cmake/modules/ClangClCompileRules.cmake b/cmake/modules/ClangClCompileRules.cmake
new file mode 100644
index 000000000..9c2a4cd2d
--- /dev/null
+++ b/cmake/modules/ClangClCompileRules.cmake
@@ -0,0 +1,8 @@
+
+# clang-cl interprets paths starting with /U as macro undefines, so we need to
+# put a -- before the input file path to force it to be treated as a path.
+string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_C_COMPILE_OBJECT "${CMAKE_C_COMPILE_OBJECT}")
+string(REPLACE "-c <SOURCE>" "-c -- <SOURCE>" CMAKE_CXX_COMPILE_OBJECT "${CMAKE_CXX_COMPILE_OBJECT}")
+
+set(CMAKE_C_LINK_EXECUTABLE "<CMAKE_C_COMPILER> <FLAGS> <CMAKE_C_LINK_FLAGS> <LINK_FLAGS> <OBJECTS> -o <TARGET> <LINK_LIBRARIES>")
+
diff --git a/cmake/modules/DispatchCompilerWarnings.cmake b/cmake/modules/DispatchCompilerWarnings.cmake
index fa3b2b571..a7b31c818 100644
--- a/cmake/modules/DispatchCompilerWarnings.cmake
+++ b/cmake/modules/DispatchCompilerWarnings.cmake
@@ -74,5 +74,6 @@ else()
       add_compile_options(-Wno-int-conversion)
       add_compile_options(-Wno-shorten-64-to-32)
     endif()
+    add_compile_options(-Wno-error=assign-enum)
   endmacro()
 endif()
diff --git a/cmake/modules/DispatchWindowsSupport.cmake b/cmake/modules/DispatchWindowsSupport.cmake
new file mode 100644
index 000000000..eed28a2f1
--- /dev/null
+++ b/cmake/modules/DispatchWindowsSupport.cmake
@@ -0,0 +1,74 @@
+
+function(dispatch_windows_arch_spelling arch var)
+  if(${arch} STREQUAL i686)
+    set(${var} x86 PARENT_SCOPE)
+  elseif(${arch} STREQUAL x86_64)
+    set(${var} x64 PARENT_SCOPE)
+  elseif(${arch} STREQUAL armv7)
+    set(${var} arm PARENT_SCOPE)
+  elseif(${arch} STREQUAL aarch64)
+    set(${var} arm64 PARENT_SCOPE)
+  else()
+    message(FATAL_ERROR "do not know MSVC spelling for ARCH: `${arch}`")
+  endif()
+endfunction()
+
+function(dispatch_verify_windows_environment_variables)
+  set(VCToolsInstallDir $ENV{VCToolsInstallDir})
+  set(UniversalCRTSdkDir $ENV{UniversalCRTSdkDir})
+  set(UCRTVersion $ENV{UCRTVersion})
+
+  if("${VCToolsInstallDir}" STREQUAL "")
+    message(SEND_ERROR "VCToolsInstallDir environment variable must be set")
+  endif()
+  if("${UniversalCRTSdkDir}" STREQUAL "")
+    message(SEND_ERROR "UniversalCRTSdkDir environment variable must be set")
+  endif()
+  if("${UCRTVersion}" STREQUAL "")
+    message(SEND_ERROR "UCRTVersion environment variable must be set")
+  endif()
+endfunction()
+
+function(dispatch_windows_include_for_arch arch var)
+  dispatch_verify_windows_environment_variables()
+
+  set(paths
+        "$ENV{VCToolsInstallDir}/include"
+        "$ENV{UniversalCRTSdkDir}/Include/$ENV{UCRTVersion}/ucrt"
+        "$ENV{UniversalCRTSdkDir}/Include/$ENV{UCRTVersion}/shared"
+        "$ENV{UniversalCRTSdkDir}/Include/$ENV{UCRTVersion}/um")
+  set(${var} ${paths} PARENT_SCOPE)
+endfunction()
+
+function(dispatch_windows_lib_for_arch arch var)
+  dispatch_verify_windows_environment_variables()
+  dispatch_windows_arch_spelling(${arch} ARCH)
+
+  set(paths)
+  if(${ARCH} STREQUAL x86)
+    list(APPEND paths "$ENV{VCToolsInstallDir}/Lib")
+  else()
+    list(APPEND paths "$ENV{VCToolsInstallDir}/Lib/${ARCH}")
+  endif()
+  list(APPEND paths
+          "$ENV{UniversalCRTSdkDir}/Lib/$ENV{UCRTVersion}/ucrt/${ARCH}"
+          "$ENV{UniversalCRTSdkDir}/Lib/$ENV{UCRTVersion}/um/${ARCH}")
+  set(${var} ${paths} PARENT_SCOPE)
+endfunction()
+
+function(dispatch_windows_generate_sdk_vfs_overlay flags)
+  dispatch_verify_windows_environment_variables()
+
+  get_filename_component(VCToolsInstallDir $ENV{VCToolsInstallDir} ABSOLUTE)
+  get_filename_component(UniversalCRTSdkDir $ENV{UniversalCRTSdkDir} ABSOLUTE)
+  set(UCRTVersion $ENV{UCRTVersion})
+
+  # TODO(compnerd) use a target to avoid re-creating this file all the time
+  configure_file("${CMAKE_SOURCE_DIR}/utils/WindowsSDKVFSOverlay.yaml.in"
+                 "${CMAKE_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
+                 @ONLY)
+
+  set(${flags}
+      -ivfsoverlay;"${CMAKE_BINARY_DIR}/windows-sdk-vfs-overlay.yaml"
+      PARENT_SCOPE)
+endfunction()
diff --git a/dispatch/base.h b/dispatch/base.h
index 0c5f8c98e..bd01f481c 100644
--- a/dispatch/base.h
+++ b/dispatch/base.h
@@ -128,15 +128,20 @@
 #endif
 #endif
 
-#if TARGET_OS_WIN32 && defined(__DISPATCH_BUILDING_DISPATCH__) && \
-		defined(__cplusplus)
-#define DISPATCH_EXPORT extern "C" extern __declspec(dllexport)
-#elif TARGET_OS_WIN32 && defined(__DISPATCH_BUILDING_DISPATCH__)
+#if defined(_WIN32)
+#if defined(__DISPATCH_BUILDING_DISPATCH__)
+#if defined(__cplusplus)
+#define DISPATCH_EXPORT extern "C" __declspec(dllexport)
+#else
 #define DISPATCH_EXPORT extern __declspec(dllexport)
-#elif TARGET_OS_WIN32 && defined(__cplusplus)
-#define DISPATCH_EXPORT extern "C" extern __declspec(dllimport)
-#elif TARGET_OS_WIN32
+#endif
+#else
+#if defined(__cplusplus)
+#define DISPATCH_EXPORT extern "C" __declspec(dllimport)
+#else
 #define DISPATCH_EXPORT extern __declspec(dllimport)
+#endif
+#endif
 #elif __GNUC__
 #define DISPATCH_EXPORT extern __attribute__((visibility("default")))
 #else
@@ -203,7 +208,7 @@
 #endif
 #endif
 
-#if __has_feature(objc_fixed_enum) || __has_extension(cxx_strong_enums)
+#if __has_feature(objc_fixed_enum) || __has_extension(cxx_strong_enums) || defined(_WIN32)
 #define DISPATCH_ENUM(name, type, ...) \
 		typedef enum : type { __VA_ARGS__ } name##_t
 #else
diff --git a/dispatch/data.h b/dispatch/data.h
index 33a0c9d51..825066918 100644
--- a/dispatch/data.h
+++ b/dispatch/data.h
@@ -62,15 +62,10 @@ DISPATCH_EXPORT struct dispatch_data_s _dispatch_data_empty;
 #define DISPATCH_DATA_DESTRUCTOR_DEFAULT NULL
 
 #ifdef __BLOCKS__
-#if !TARGET_OS_WIN32
 /*! @parseOnly */
 #define DISPATCH_DATA_DESTRUCTOR_TYPE_DECL(name) \
 	DISPATCH_EXPORT const dispatch_block_t _dispatch_data_destructor_##name
 #else
-#define DISPATCH_DATA_DESTRUCTOR_TYPE_DECL(name) \
-	DISPATCH_EXPORT dispatch_block_t _dispatch_data_destructor_##name
-#endif
-#else
 #define DISPATCH_DATA_DESTRUCTOR_TYPE_DECL(name) \
 	DISPATCH_EXPORT const dispatch_function_t \
 	_dispatch_data_destructor_##name
diff --git a/dispatch/dispatch.h b/dispatch/dispatch.h
index d36259c75..190122605 100644
--- a/dispatch/dispatch.h
+++ b/dispatch/dispatch.h
@@ -26,7 +26,9 @@
 #include <os/availability.h>
 #include <TargetConditionals.h>
 #include <os/base.h>
-#elif defined(__linux__) || defined(__FreeBSD__)
+#elif defined(_WIN32)
+#include <os/generic_win_base.h>
+#elif defined(__unix__)
 #include <os/generic_unix_base.h>
 #endif
 
diff --git a/dispatch/io.h b/dispatch/io.h
index a9e6892e5..db9733d82 100644
--- a/dispatch/io.h
+++ b/dispatch/io.h
@@ -50,7 +50,11 @@ __BEGIN_DECLS
  * @typedef dispatch_fd_t
  * Native file descriptor type for the platform.
  */
+#if defined(_WIN32)
+typedef intptr_t dispatch_fd_t;
+#else
 typedef int dispatch_fd_t;
+#endif
 
 /*!
  * @functiongroup Dispatch I/O Convenience API
diff --git a/dispatch/object.h b/dispatch/object.h
index 3ff36c2d3..3a31b1069 100644
--- a/dispatch/object.h
+++ b/dispatch/object.h
@@ -121,7 +121,7 @@ typedef union {
 #ifndef DISPATCH_DATA_DECL
 #define DISPATCH_DATA_DECL(name) OS_OBJECT_DECL_SWIFT(name)
 #endif // DISPATCH_DATA_DECL
-#elif !TARGET_OS_WIN32
+#else
 /*! @parseOnly */
 #define DISPATCH_SOURCE_DECL(name) \
 		DISPATCH_DECL(name);
@@ -131,12 +131,6 @@ typedef union {
 #define DISPATCH_SOURCE_TYPE_DECL(name) \
 		DISPATCH_EXPORT const struct dispatch_source_type_s \
 		_dispatch_source_type_##name
-#else
-#define DISPATCH_SOURCE_DECL(name) \
-		DISPATCH_DECL(name);
-#define DISPATCH_SOURCE_TYPE_DECL(name) \
-		DISPATCH_EXPORT struct dispatch_source_type_s _dispatch_source_type_##name
-#define DISPATCH_DATA_DECL(name) DISPATCH_DECL(name)
 #endif
 
 #ifdef __BLOCKS__
diff --git a/dispatch/source.h b/dispatch/source.h
index 6992d4226..35bb03fca 100644
--- a/dispatch/source.h
+++ b/dispatch/source.h
@@ -31,7 +31,7 @@
 #include <mach/message.h>
 #endif
 
-#if !TARGET_OS_WIN32
+#if !defined(_WIN32)
 #include <sys/signal.h>
 #endif
 
diff --git a/os/generic_win_base.h b/os/generic_win_base.h
new file mode 100644
index 000000000..8b4c6138a
--- /dev/null
+++ b/os/generic_win_base.h
@@ -0,0 +1,132 @@
+/*
+ * This source file is part of the Swift.org open source project
+ *
+ * Copyright (c) 2015 Apple Inc. and the Swift project authors
+ *
+ * Licensed under Apache License v2.0 with Runtime Library Exception
+ *
+ * See http://swift.org/LICENSE.txt for license information
+ * See http://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+ *
+ */
+
+#ifndef __OS_GENERIC_WIN_BASE__
+#define __OS_GENERIC_WIN_BASE__
+
+// Unices provide `roundup` via sys/param.h
+#define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y))
+// Unices provide `MAX` via sys/param.h
+#define MAX(a,b) (((a)>(b))?(a):(b))
+// Unices provide `MIN` via sys/param.h
+#define MIN(a,b) (((a)<(b))?(a):(b))
+// Unices provide `howmany` via sys/param.h
+#define howmany(x, y)  (((x) + ((y) - 1)) / (y))
+
+typedef int mode_t;
+typedef void pthread_attr_t;
+
+#if defined(__cplusplus)
+#define __BEGIN_DECLS extern "C" {
+#define __END_DECLS }
+#else
+#define __BEGIN_DECLS
+#define __END_DECLS
+#endif
+
+#ifndef API_AVAILABLE
+#define API_AVAILABLE(...)
+#endif
+#ifndef API_DEPRECATED
+#define API_DEPRECATED(...)
+#endif
+#ifndef API_UNAVAILABLE
+#define API_UNAVAILABLE(...)
+#endif
+#ifndef API_DEPRECATED_WITH_REPLACEMENT
+#define API_DEPRECATED_WITH_REPLACEMENT(...)
+#endif
+
+#if !defined(__has_attribute)
+#define __has_attribute(attibute) 0
+#endif
+
+#if !defined(__has_builtin)
+#define __has_builtin(builtin) 0
+#endif
+
+#if !defined(__has_feature)
+#define __has_feature(feature) 0
+#endif
+
+#if __has_builtin(__builtin_expect)
+#define OS_EXPECT(expression, value) __builtin_expect((expression), (value))
+#else
+#define OS_EXPECT(expression, value) (expression)
+#endif
+
+#if __has_attribute(__unused__)
+#define OS_UNUSED __attribute__((__unused__))
+#else
+#define OS_UNUSED
+#endif
+
+#ifndef os_likely
+#define os_likely(expression) OS_EXPECT(!!(expression), 1)
+#endif
+#ifndef os_unlikely
+#define os_unlikely(expression) OS_EXPECT(!!(expression), 0)
+#endif
+
+#if __has_feature(assume_nonnull)
+#define OS_ASSUME_NONNULL_BEGIN _Pragma("clang assume_nonnull begin")
+#define OS_ASSUME_NONNULL_END   _Pragma("clang assume_nonnull end")
+#else
+#define OS_ASSUME_NONNULL_BEGIN
+#define OS_ASSUME_NONNULL_END
+#endif
+
+#if __has_builtin(__builtin_assume)
+#define OS_COMPILER_CAN_ASSUME(expr) __builtin_assume(expr)
+#else
+#define OS_COMPILER_CAN_ASSUME(expr) ((void)(expr))
+#endif
+
+#if __has_feature(attribute_availability_swift)
+// equivalent to __SWIFT_UNAVAILABLE from Availability.h
+#define OS_SWIFT_UNAVAILABLE(msg)                                              \
+  __attribute__((__availability__(swift, unavailable, message = msg)))
+#else
+#define OS_SWIFT_UNAVAILABLE(msg)
+#endif
+
+#define __OS_STRINGIFY(s) #s
+#define OS_STRINGIFY(s) __OS_STRINGIFY(s)
+
+#if __has_feature(objc_fixed_enum) || __has_extension(cxx_strong_enums)
+#define OS_ENUM(name, type, ...) typedef enum : type { __VA_ARGS__ } name##_t
+#else
+#define OS_ENUM(name, type, ...)                                               \
+  enum { __VA_ARGS__ };                                                        \
+  typedef type name##_t
+#endif
+
+#ifdef OS_EXPORT
+#undef OS_EXPORT
+#endif
+#define OS_EXPORT __declspec(dllexport)
+
+#ifdef OS_WARN_RESULT_NEEDS_RELEASE
+#undef OS_WARN_RESULT_NEEDS_RELEASE
+#endif
+
+#ifdef OS_WARN_RESULT
+#undef OS_WARN_RESULT
+#endif
+#define OS_WARN_RESULT
+
+#ifdef OS_NOTHROW
+#undef OS_NOTHROW
+#endif
+#define OS_NOTHROW
+
+#endif
diff --git a/os/object.h b/os/object.h
index 3666c332a..1ad1158c5 100644
--- a/os/object.h
+++ b/os/object.h
@@ -26,7 +26,9 @@
 #include <os/availability.h>
 #include <TargetConditionals.h>
 #include <os/base.h>
-#elif defined(__linux__) || defined(__FreeBSD__)
+#elif defined(_WIN32)
+#include <os/generic_win_base.h>
+#elif defined(__unix__)
 #include <os/generic_unix_base.h>
 #endif
 
diff --git a/private/data_private.h b/private/data_private.h
index 364a8ffe0..a92215720 100644
--- a/private/data_private.h
+++ b/private/data_private.h
@@ -182,15 +182,9 @@ dispatch_data_make_memory_entry(dispatch_data_t data);
  */
 typedef const struct dispatch_data_format_type_s *dispatch_data_format_type_t;
 
-#if !TARGET_OS_WIN32
 #define DISPATCH_DATA_FORMAT_TYPE_DECL(name) \
 	DISPATCH_EXPORT const struct dispatch_data_format_type_s \
 	_dispatch_data_format_type_##name
-#else
-#define DISPATCH_DATA_FORMAT_TYPE_DECL(name) \
-	DISPATCH_EXPORT struct dispatch_data_format_type_s \
-	_dispatch_data_format_type_##name
-#endif
 
 /*!
  * @const DISPATCH_DATA_FORMAT_TYPE_NONE
diff --git a/private/layout_private.h b/private/layout_private.h
index 0c0cd942d..81bcabd54 100644
--- a/private/layout_private.h
+++ b/private/layout_private.h
@@ -28,7 +28,6 @@
 
 __BEGIN_DECLS
 
-#if !TARGET_OS_WIN32
 API_AVAILABLE(macos(10.6), ios(4.0))
 DISPATCH_EXPORT const struct dispatch_queue_offsets_s {
 	// always add new fields at the end
@@ -51,7 +50,6 @@ DISPATCH_EXPORT const struct dispatch_queue_offsets_s {
 	const uint16_t dqo_priority;
 	const uint16_t dqo_priority_size;
 } dispatch_queue_offsets;
-#endif
 
 #if DISPATCH_LAYOUT_SPI
 
diff --git a/private/private.h b/private/private.h
index ee2b26e43..7fba396e1 100644
--- a/private/private.h
+++ b/private/private.h
@@ -32,7 +32,9 @@
 #include <os/availability.h>
 #include <TargetConditionals.h>
 #include <os/base.h>
-#elif defined(__linux__) || defined(__FreeBSD__)
+#elif defined(_WIN32)
+#include <os/generic_win_base.h>
+#elif defined(__unix__)
 #include <os/generic_unix_base.h>
 #endif
 
@@ -44,7 +46,9 @@
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #include <unistd.h>
 #endif
+#if !defined(_WIN32)
 #include <pthread.h>
+#endif
 #if TARGET_OS_MAC
 #include <pthread/qos.h>
 #endif
diff --git a/src/BlocksRuntime/runtime.c b/src/BlocksRuntime/runtime.c
index da85f7131..b8c4b33bf 100644
--- a/src/BlocksRuntime/runtime.c
+++ b/src/BlocksRuntime/runtime.c
@@ -172,7 +172,7 @@ static void _Block_release_object_default(const void *ptr) {
 }
 
 static void _Block_assign_weak_default(const void *ptr, void *dest) {
-#if !TARGET_OS_WIN32
+#if !defined(_WIN32)
     *(long *)dest = (long)ptr;
 #else
     *(void **)dest = (void *)ptr;
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index c9c55b5f2..a7cd89bbd 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -47,14 +47,23 @@ add_library(dispatch
               shims/atomic_sfb.h
               shims/getprogname.h
               shims/hw_config.h
-              shims/generic_unix_stubs.c
-              shims/generic_unix_stubs.h
               shims/lock.c
               shims/lock.h
               shims/perfmon.h
               shims/time.h
               shims/tsd.h
               shims/yield.h)
+if(UNIX)
+  target_sources(dispatch
+                 PRIVATE
+                   shims/generic_unix_stubs.c
+                   shims/generic_unix_stubs.h)
+elseif(WIN32)
+  target_sources(dispatch
+                 PRIVATE
+                   shims/generic_win_stubs.c
+                   shims/generic_win_stubs.h)
+endif()
 if(DISPATCH_USE_INTERNAL_WORKQUEUE)
   target_sources(dispatch
                  PRIVATE
@@ -130,6 +139,11 @@ if(WITH_BLOCKS_RUNTIME)
                              SYSTEM BEFORE PRIVATE
                                "${WITH_BLOCKS_RUNTIME}")
 endif()
+if(WIN32)
+  target_compile_definitions(dispatch
+                             PRIVATE
+                               _CRT_NONSTDC_NO_WARNINGS)
+endif()
 if("${CMAKE_C_SIMULATE_ID}" STREQUAL "MSVC")
   target_compile_options(dispatch PRIVATE /EHsc-)
 else()
diff --git a/src/block.cpp b/src/block.cpp
index 8f8113af3..a46b55113 100644
--- a/src/block.cpp
+++ b/src/block.cpp
@@ -69,7 +69,9 @@ struct dispatch_block_private_data_s {
 	{
 		// copy constructor, create copy with retained references
 		if (dbpd_voucher) voucher_retain(dbpd_voucher);
-		if (o.dbpd_block) dbpd_block = _dispatch_Block_copy(o.dbpd_block);
+		if (o.dbpd_block) {
+			dbpd_block = reinterpret_cast<dispatch_block_t>(_dispatch_Block_copy(o.dbpd_block));
+		}
 		_dispatch_block_private_data_debug("copy from %p, block: %p from %p",
 				&o, dbpd_block, o.dbpd_block);
 		if (!o.dbpd_magic) return; // No group in initial copy of stack object
@@ -98,11 +100,11 @@ _dispatch_block_create(dispatch_block_flags_t flags, voucher_t voucher,
 		pthread_priority_t pri, dispatch_block_t block)
 {
 	struct dispatch_block_private_data_s dbpds(flags, voucher, pri, block);
-	return _dispatch_Block_copy(^{
+	return reinterpret_cast<dispatch_block_t>(_dispatch_Block_copy(^{
 		// Capture stack object: invokes copy constructor (17094902)
 		(void)dbpds;
 		_dispatch_block_invoke_direct(&dbpds);
-	});
+	}));
 }
 
 extern "C" {
diff --git a/src/event/event_config.h b/src/event/event_config.h
index 871a3e036..fda5d22c0 100644
--- a/src/event/event_config.h
+++ b/src/event/event_config.h
@@ -29,6 +29,9 @@
 #	include <sys/event.h>
 #	define DISPATCH_EVENT_BACKEND_EPOLL 0
 #	define DISPATCH_EVENT_BACKEND_KEVENT 1
+#elif defined(_WIN32)
+#	define DISPATCH_EVENT_BACKEND_EPOLL 0
+#	define DISPATCH_EVENT_BACKEND_KEVENT 0
 #else
 #	error unsupported event loop
 #endif
diff --git a/src/init.c b/src/init.c
index 4ef733d7b..00a04d3bc 100644
--- a/src/init.c
+++ b/src/init.c
@@ -37,7 +37,7 @@ DISPATCH_NOTHROW __attribute__((constructor))
 void
 _libdispatch_init(void);
 
-DISPATCH_EXPORT DISPATCH_NOTHROW
+DISPATCH_NOTHROW
 void
 _libdispatch_init(void)
 {
@@ -45,6 +45,7 @@ _libdispatch_init(void)
 }
 #endif
 
+#if !defined(_WIN32)
 DISPATCH_EXPORT DISPATCH_NOTHROW
 void
 dispatch_atfork_prepare(void)
@@ -96,6 +97,7 @@ _dispatch_sigmask(void)
 	r |= pthread_sigmask(SIG_BLOCK, &mask, NULL);
 	return dispatch_assume_zero(r);
 }
+#endif
 
 #pragma mark -
 #pragma mark dispatch_globals
@@ -111,7 +113,11 @@ void (*_dispatch_end_NSAutoReleasePool)(void *);
 
 #if DISPATCH_USE_THREAD_LOCAL_STORAGE
 __thread struct dispatch_tsd __dispatch_tsd;
+#if defined(_WIN32)
+DWORD __dispatch_tsd_key;
+#else
 pthread_key_t __dispatch_tsd_key;
+#endif
 #elif !DISPATCH_USE_DIRECT_TSD
 pthread_key_t dispatch_queue_key;
 pthread_key_t dispatch_frame_key;
@@ -690,26 +696,57 @@ _dispatch_logv_init(void *context DISPATCH_UNUSED)
 			log_to_file = true;
 		} else if (strcmp(e, "stderr") == 0) {
 			log_to_file = true;
+#if defined(_WIN32)
+			dispatch_logfile = _fileno(stderr);
+#else
 			dispatch_logfile = STDERR_FILENO;
+#endif
 		}
 	}
 	if (!dispatch_log_disabled) {
 		if (log_to_file && dispatch_logfile == -1) {
+#if defined(_WIN32)
+			char path[MAX_PATH + 1] = {0};
+			DWORD dwLength = GetTempPathA(MAX_PATH, path);
+			dispatch_assert(dwLength <= MAX_PATH + 1);
+			snprintf(&path[dwLength], MAX_PATH - dwLength, "libdispatch.%d.log",
+					GetCurrentProcessId());
+			dispatch_logfile = _open(path, O_WRONLY | O_APPEND | O_CREAT, 0666);
+#else
 			char path[PATH_MAX];
 			snprintf(path, sizeof(path), "/var/tmp/libdispatch.%d.log",
 					getpid());
 			dispatch_logfile = open(path, O_WRONLY | O_APPEND | O_CREAT |
 					O_NOFOLLOW | O_CLOEXEC, 0666);
+#endif
 		}
 		if (dispatch_logfile != -1) {
 			struct timeval tv;
+#if defined(_WIN32)
+			DWORD dwTime = GetTickCount();
+			tv.tv_sec = dwTime / 1000;
+			tv.tv_usec = 1000 * (dwTime % 1000);
+#else
 			gettimeofday(&tv, NULL);
+#endif
 #if DISPATCH_DEBUG
 			dispatch_log_basetime = _dispatch_absolute_time();
 #endif
+#if defined(_WIN32)
+			FILE *pLogFile = _fdopen(dispatch_logfile, "w");
+
+			char szProgramName[MAX_PATH + 1] = {0};
+			GetModuleFileNameA(NULL, szProgramName, MAX_PATH);
+
+			fprintf(pLogFile, "=== log file opened for %s[%lu] at "
+					"%ld.%06u ===\n", szProgramName, GetCurrentProcessId(),
+					tv.tv_sec, (int)tv.tv_usec);
+			fclose(pLogFile);
+#else
 			dprintf(dispatch_logfile, "=== log file opened for %s[%u] at "
 					"%ld.%06u ===\n", getprogname() ?: "", getpid(),
 					tv.tv_sec, (int)tv.tv_usec);
+#endif
 		}
 	}
 }
@@ -721,7 +758,12 @@ _dispatch_log_file(char *buf, size_t len)
 
 	buf[len++] = '\n';
 retry:
+#if defined(_WIN32)
+	dispatch_assert(len <= UINT_MAX);
+	r = _write(dispatch_logfile, buf, (unsigned int)len);
+#else
 	r = write(dispatch_logfile, buf, len);
+#endif
 	if (slowpath(r == -1) && errno == EINTR) {
 		goto retry;
 	}
@@ -765,6 +807,36 @@ _dispatch_vsyslog(const char *msg, va_list ap)
 		free(str);
 	}
 }
+#elif defined(_WIN32)
+static inline void
+_dispatch_syslog(const char *msg)
+{
+  OutputDebugStringA(msg);
+}
+
+static inline void
+_dispatch_vsyslog(const char *msg, va_list ap)
+{
+  va_list argp;
+
+  va_copy(argp, ap);
+
+  int length = _vscprintf(msg, ap);
+  if (length == -1)
+    return;
+
+  char *buffer = malloc((size_t)length + 1);
+  if (buffer == NULL)
+    return;
+
+  _vsnprintf(buffer, (size_t)length + 1, msg, argp);
+
+  va_end(argp);
+
+  _dispatch_syslog(buffer);
+
+  free(buffer);
+}
 #else // DISPATCH_USE_SIMPLE_ASL
 static inline void
 _dispatch_syslog(const char *msg)
diff --git a/src/internal.h b/src/internal.h
index 84505e527..126f4c699 100644
--- a/src/internal.h
+++ b/src/internal.h
@@ -148,9 +148,7 @@
 #include <dispatch/semaphore.h>
 #include <dispatch/once.h>
 #include <dispatch/data.h>
-#if !TARGET_OS_WIN32
 #include <dispatch/io.h>
-#endif
 
 #if defined(__OBJC__) || defined(__cplusplus)
 #define DISPATCH_PURE_C 0
@@ -159,7 +157,9 @@
 #endif
 
 /* private.h must be included last to avoid picking up installed headers. */
+#if !defined(_WIN32)
 #include <pthread.h>
+#endif
 #include "os/object_private.h"
 #include "queue_private.h"
 #include "source_private.h"
@@ -167,16 +167,14 @@
 #include "data_private.h"
 #include "os/voucher_private.h"
 #include "os/voucher_activity_private.h"
-#if !TARGET_OS_WIN32
 #include "io_private.h"
-#endif
 #include "layout_private.h"
 #include "benchmark.h"
 #include "private.h"
 
 /* SPI for Libsystem-internal use */
 DISPATCH_EXPORT DISPATCH_NOTHROW void libdispatch_init(void);
-#if !TARGET_OS_WIN32
+#if !defined(_WIN32)
 DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_prepare(void);
 DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_parent(void);
 DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_child(void);
@@ -244,10 +242,12 @@ DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_child(void);
 #endif
 
 #include <sys/stat.h>
+#include <sys/queue.h>
 
-#if !TARGET_OS_WIN32
+#if defined(_WIN32)
+#include <time.h>
+#else
 #include <sys/mount.h>
-#include <sys/queue.h>
 #ifdef __ANDROID__
 #include <linux/sysctl.h>
 #else
@@ -283,13 +283,19 @@ DISPATCH_EXPORT DISPATCH_NOTHROW void dispatch_atfork_child(void);
 #include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
+#if defined(_WIN32)
+#define _CRT_RAND_S
+#endif
 #include <stdlib.h>
 #include <string.h>
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #include <unistd.h>
 #endif
+#if defined(_WIN32)
+#include <io.h>
+#endif
 
-#if __GNUC__
+#if defined(__GNUC__) || defined(__clang__)
 #define DISPATCH_NOINLINE __attribute__((__noinline__))
 #define DISPATCH_USED __attribute__((__used__))
 #define DISPATCH_UNUSED __attribute__((__unused__))
@@ -420,7 +426,7 @@ void _dispatch_abort(size_t line, long val);
 #endif
 #endif // DISPATCH_USE_SIMPLE_ASL
 
-#if !DISPATCH_USE_SIMPLE_ASL && !DISPATCH_USE_OS_DEBUG_LOG && !TARGET_OS_WIN32
+#if !DISPATCH_USE_SIMPLE_ASL && !DISPATCH_USE_OS_DEBUG_LOG && !defined(_WIN32)
 #include <syslog.h>
 #endif
 
@@ -513,7 +519,7 @@ _dispatch_assert_zero(long e, size_t line)
 	})
 #else
 static inline long
-_dispatch_assume(long e, long line)
+_dispatch_assume(long e, unsigned long line)
 {
 	if (!e) _dispatch_bug(line, e);
 	return e;
@@ -538,7 +544,7 @@ _dispatch_assume(long e, long line)
 	})
 #else
 static inline long
-_dispatch_assume_zero(long e, long line)
+_dispatch_assume_zero(long e, unsigned long line)
 {
 	if (e) _dispatch_bug(line, e);
 	return e;
@@ -604,7 +610,9 @@ void *_dispatch_calloc(size_t num_items, size_t size);
 const char *_dispatch_strdup_if_mutable(const char *str);
 void _dispatch_vtable_init(void);
 char *_dispatch_get_build(void);
+#if !defined(_WIN32)
 int _dispatch_sigmask(void);
+#endif
 
 uint64_t _dispatch_timeout(dispatch_time_t when);
 uint64_t _dispatch_time_nanoseconds_since_epoch(dispatch_time_t when);
@@ -913,6 +921,18 @@ _dispatch_ktrace_impl(uint32_t code, uint64_t a, uint64_t b,
 		_dispatch_hardware_crash(); \
 	} while (0)
 
+#if defined(_WIN32)
+#define _dispatch_client_assert_fail(fmt, ...)  do { \
+		char *_msg = NULL; \
+		int _length = _scprintf("%s" fmt, DISPATCH_ASSERTION_FAILED_MESSAGE, ##__VA_ARGS__); \
+		dispatch_assert(_length != -1); \
+		_msg = (char *)malloc((unsigned)_length + 1); \
+		dispatch_assert(_msg); \
+		_snprintf(_msg, (unsigned)_length, "%s" fmt, DISPATCH_ASSERTION_FAILED_MESSAGE, ##__VA_ARGS__); \
+		_dispatch_assert_crash(_msg); \
+		free(_msg); \
+	} while (0)
+#else
 #define _dispatch_client_assert_fail(fmt, ...)  do { \
 		char *_msg = NULL; \
 		asprintf(&_msg, "%s" fmt, DISPATCH_ASSERTION_FAILED_MESSAGE, \
@@ -920,6 +940,7 @@ _dispatch_ktrace_impl(uint32_t code, uint64_t a, uint64_t b,
 		_dispatch_assert_crash(_msg); \
 		free(_msg); \
 	} while (0)
+#endif
 
 #define DISPATCH_NO_VOUCHER ((voucher_t)(void*)~0ul)
 #define DISPATCH_NO_PRIORITY ((pthread_priority_t)~0ul)
@@ -967,9 +988,7 @@ extern int _dispatch_kevent_workqueue_enabled;
 #include "mach_internal.h"
 #include "voucher_internal.h"
 #include "data_internal.h"
-#if !TARGET_OS_WIN32
 #include "io_internal.h"
-#endif
 #include "inline_internal.h"
 #include "firehose/firehose_internal.h"
 
diff --git a/src/introspection.c b/src/introspection.c
index 1332adfd7..1bb095d1c 100644
--- a/src/introspection.c
+++ b/src/introspection.c
@@ -137,7 +137,7 @@ _dispatch_introspection_thread_add(void)
 	_dispatch_unfair_lock_unlock(&_dispatch_introspection.threads_lock);
 }
 
-static void
+static DISPATCH_TSD_DTOR_CC void
 _dispatch_introspection_thread_remove(void *ctxt)
 {
 	dispatch_introspection_thread_t dit = ctxt;
diff --git a/src/io.c b/src/io.c
index 6ec45d8a6..fbb03736f 100644
--- a/src/io.c
+++ b/src/io.c
@@ -30,6 +30,15 @@
 #endif
 
 #ifndef PAGE_SIZE
+#if defined(_WIN32)
+static DWORD
+getpagesize(void)
+{
+	SYSTEM_INFO siInfo;
+	GetSystemInfo(&siInfo);
+	return siInfo.dwPageSize;
+}
+#endif
 #define PAGE_SIZE ((size_t)getpagesize())
 #endif
 
@@ -366,12 +375,23 @@ dispatch_io_create(dispatch_io_type_t type, dispatch_fd_t fd,
 			err = _dispatch_io_validate_type(channel, fd_entry->stat.mode);
 		}
 		if (!err && type == DISPATCH_IO_RANDOM) {
+#if defined(_WIN32)
+			LARGE_INTEGER liPosition;
+			LARGE_INTEGER liDistance = {};
+			if (!SetFilePointerEx((HANDLE)fd_entry->fd, liDistance, &liPosition, FILE_CURRENT)) {
+				err = (int)GetLastError();
+			} else {
+				err = 0;
+				channel->f_ptr = liPosition.QuadPart;
+			}
+#else
 			off_t f_ptr;
 			_dispatch_io_syscall_switch_noerr(err,
 				f_ptr = lseek(fd_entry->fd, 0, SEEK_CUR),
 				case 0: channel->f_ptr = f_ptr; break;
 				default: (void)dispatch_assume_zero(err); break;
 			);
+#endif
 		}
 		channel->err = err;
 		_dispatch_fd_entry_retain(fd_entry);
@@ -423,11 +443,15 @@ dispatch_io_create_with_path(dispatch_io_type_t type, const char *path,
 		int err = 0;
 		struct stat st;
 		_dispatch_io_syscall_switch_noerr(err,
+#if defined(_WIN32)
+			stat(path_data->path, &st),
+#else
 			(path_data->oflag & O_NOFOLLOW) == O_NOFOLLOW
 #if __APPLE__
 					|| (path_data->oflag & O_SYMLINK) == O_SYMLINK
 #endif
 					? lstat(path_data->path, &st) : stat(path_data->path, &st),
+#endif
 			case 0:
 				err = _dispatch_io_validate_type(channel, st.st_mode);
 				break;
@@ -528,12 +552,23 @@ dispatch_io_create_with_io(dispatch_io_type_t type, dispatch_io_t in_channel,
 						in_channel->fd_entry->stat.mode);
 			}
 			if (!err && type == DISPATCH_IO_RANDOM && in_channel->fd != -1) {
+#if defined(_WIN32)
+				LARGE_INTEGER liPosition;
+				LARGE_INTEGER liDistance = {};
+				if (!SetFilePointerEx((HANDLE)in_channel->fd_entry->fd, liDistance, &liPosition, FILE_CURRENT)) {
+					err = (int)GetLastError();
+				} else {
+					err = 0;
+					channel->f_ptr = liPosition.QuadPart;
+				}
+#else
 				off_t f_ptr;
 				_dispatch_io_syscall_switch_noerr(err,
 					f_ptr = lseek(in_channel->fd_entry->fd, 0, SEEK_CUR),
 					case 0: channel->f_ptr = f_ptr; break;
 					default: (void)dispatch_assume_zero(err); break;
 				);
+#endif
 			}
 			channel->err = err;
 			if (err) {
@@ -1075,7 +1110,11 @@ _dispatch_operation_dispose(dispatch_operation_t op,
 	}
 	// For write operations, op->buf is owned by op->buf_data
 	if (op->buf && op->direction == DOP_DIR_READ) {
+#if defined(_WIN32)
+		_aligned_free(op->buf);
+#else
 		free(op->buf);
+#endif
 	}
 	if (op->buf_data) {
 		_dispatch_io_data_release(op->buf_data);
@@ -1226,13 +1265,15 @@ _dispatch_fd_entry_unguard(dispatch_fd_entry_t fd_entry)
 	);
 }
 #else
+#if !defined(_WIN32)
 static inline void
 _dispatch_fd_entry_guard(dispatch_fd_entry_t fd_entry) { (void)fd_entry; }
+#endif
 static inline void
 _dispatch_fd_entry_unguard(dispatch_fd_entry_t fd_entry) { (void)fd_entry; }
 #endif // DISPATCH_USE_GUARDED_FD
 
-static inline int
+static inline dispatch_fd_t
 _dispatch_fd_entry_guarded_open(dispatch_fd_entry_t fd_entry, const char *path,
 		int oflag, mode_t mode) {
 #if DISPATCH_USE_GUARDED_FD
@@ -1249,11 +1290,28 @@ _dispatch_fd_entry_guarded_open(dispatch_fd_entry_t fd_entry, const char *path,
 #else
 	(void)fd_entry;
 #endif
+#if defined(_WIN32)
+	(void)mode;
+	DWORD dwDesiredAccess = 0;
+	if (oflag & _O_RDWR)
+		dwDesiredAccess = GENERIC_READ | GENERIC_WRITE;
+	else if (oflag & _O_RDONLY)
+		dwDesiredAccess = GENERIC_READ;
+	else if (oflag & _O_WRONLY)
+		dwDesiredAccess = GENERIC_WRITE;
+	DWORD dwCreationDisposition = OPEN_EXISTING;
+	if (oflag & _O_CREAT)
+		dwCreationDisposition = OPEN_ALWAYS;
+	if (oflag & _O_TRUNC)
+		dwCreationDisposition = CREATE_ALWAYS;
+	return (dispatch_fd_t)CreateFile(path, dwDesiredAccess, 0, NULL, dwCreationDisposition, 0, NULL);
+#else
 	return open(path, oflag, mode);
+#endif
 }
 
 static inline int
-_dispatch_fd_entry_guarded_close(dispatch_fd_entry_t fd_entry, int fd) {
+_dispatch_fd_entry_guarded_close(dispatch_fd_entry_t fd_entry, dispatch_fd_t fd) {
 #if DISPATCH_USE_GUARDED_FD
 	if (fd_entry->guard_flags) {
 		guardid_t guard = (uintptr_t)fd_entry;
@@ -1263,7 +1321,11 @@ _dispatch_fd_entry_guarded_close(dispatch_fd_entry_t fd_entry, int fd) {
 	(void)fd_entry;
 #endif
 	{
+#if defined(_WIN32)
+		return CloseHandle((HANDLE)fd);
+#else
 		return close(fd);
+#endif
 	}
 }
 
@@ -1337,6 +1399,24 @@ _dispatch_fd_entry_create_with_fd(dispatch_fd_t fd, uintptr_t hash)
 			"com.apple.libdispatch-io.barrierq", NULL);
 	fd_entry->barrier_group = dispatch_group_create();
 	dispatch_async(fd_entry->barrier_queue, ^{
+#if defined(_WIN32)
+		DWORD dwType = GetFileType((HANDLE)fd);
+		if (dwType == FILE_TYPE_PIPE) {
+			unsigned long value = 1;
+			int result = ioctlsocket((SOCKET)fd, (long)FIONBIO, &value);
+			(void)dispatch_assume_zero(result);
+			_dispatch_stream_init(fd_entry,
+				_dispatch_get_root_queue(DISPATCH_QOS_DEFAULT, false));
+		} else {
+			dispatch_suspend(fd_entry->barrier_queue);
+			dispatch_once_f(&_dispatch_io_devs_lockq_pred, NULL,
+					_dispatch_io_devs_lockq_init);
+			dispatch_async(_dispatch_io_devs_lockq, ^{
+				_dispatch_disk_init(fd_entry, 0);
+				dispatch_resume(fd_entry->barrier_queue);
+			});
+		}
+#else
 		_dispatch_fd_entry_debug("stat", fd_entry);
 		int err, orig_flags, orig_nosigpipe = -1;
 		struct stat st;
@@ -1404,6 +1484,7 @@ _dispatch_fd_entry_create_with_fd(dispatch_fd_t fd, uintptr_t hash)
 		}
 		fd_entry->orig_flags = orig_flags;
 		fd_entry->orig_nosigpipe = orig_nosigpipe;
+#endif
 	});
 	// This is the first item run when the close queue is resumed, indicating
 	// that all channels associated with this entry have been closed and that
@@ -1434,6 +1515,7 @@ _dispatch_fd_entry_create_with_fd(dispatch_fd_t fd, uintptr_t hash)
 		dispatch_release(fd_entry->barrier_queue);
 		_dispatch_fd_entry_debug("barrier group release", fd_entry);
 		dispatch_release(fd_entry->barrier_group);
+#if !defined(_WIN32)
 		if (fd_entry->orig_flags != -1) {
 			_dispatch_io_syscall(
 				fcntl(fd, F_SETFL, fd_entry->orig_flags)
@@ -1445,6 +1527,7 @@ _dispatch_fd_entry_create_with_fd(dispatch_fd_t fd, uintptr_t hash)
 				fcntl(fd, F_SETNOSIGPIPE, fd_entry->orig_nosigpipe)
 			);
 		}
+#endif
 #endif
 		_dispatch_fd_entry_unguard(fd_entry);
 		if (fd_entry->convenience_channel) {
@@ -1465,7 +1548,11 @@ _dispatch_fd_entry_create_with_path(dispatch_io_path_data_t path_data,
 			path_data->channel->queue);
 	_dispatch_fd_entry_debug("create: path %s", fd_entry, path_data->path);
 	if (S_ISREG(mode)) {
+#if defined(_WIN32)
+		_dispatch_disk_init(fd_entry, 0);
+#else
 		_dispatch_disk_init(fd_entry, (dev_t)major(dev));
+#endif
 	} else {
 			_dispatch_stream_init(fd_entry,
 					_dispatch_get_root_queue(DISPATCH_QOS_DEFAULT, false));
@@ -1520,7 +1607,7 @@ _dispatch_fd_entry_open(dispatch_fd_entry_t fd_entry, dispatch_io_t channel)
 	if (fd_entry->err) {
 		return fd_entry->err;
 	}
-	int fd = -1;
+	dispatch_fd_t fd = -1;
 	int oflag = fd_entry->disk ? fd_entry->path_data->oflag & ~O_NONBLOCK :
 			fd_entry->path_data->oflag | O_NONBLOCK;
 open:
@@ -2143,6 +2230,10 @@ static void
 _dispatch_operation_advise(dispatch_operation_t op, size_t chunk_size)
 {
 	_dispatch_op_debug("advise", op);
+#if defined(_WIN32)
+	(void)op;
+	(void)chunk_size;
+#else
 	if (_dispatch_io_get_error(op, NULL, true)) return;
 #if defined(__linux__) || defined(__FreeBSD__)
 	// linux does not support fcntl (F_RDAVISE)
@@ -2186,6 +2277,7 @@ _dispatch_operation_advise(dispatch_operation_t op, size_t chunk_size)
 		default: (void)dispatch_assume_zero(err); break;
 	);
 #endif
+#endif
 }
 
 static int
@@ -2219,7 +2311,17 @@ _dispatch_operation_perform(dispatch_operation_t op)
 			} else {
 				op->buf_siz = max_buf_siz;
 			}
+#if defined(_WIN32)
+			static bool bQueried = false;
+			static SYSTEM_INFO siInfo;
+			if (!bQueried) {
+				GetNativeSystemInfo(&siInfo);
+				bQueried = true;
+			}
+			op->buf = _aligned_malloc(op->buf_siz, siInfo.dwPageSize);
+#else
 			op->buf = valloc(op->buf_siz);
+#endif
 			_dispatch_op_debug("buffer allocated", op);
 		} else if (op->direction == DOP_DIR_WRITE) {
 			// Always write the first data piece, if that is smaller than a
@@ -2257,20 +2359,51 @@ _dispatch_operation_perform(dispatch_operation_t op)
 	}
 	void *buf = op->buf + op->buf_len;
 	size_t len = op->buf_siz - op->buf_len;
+#if defined(_WIN32)
+	assert(len <= UINT_MAX && "overflow for read/write");
+	LONGLONG off = (LONGLONG)((size_t)op->offset + op->total);
+#else
 	off_t off = (off_t)((size_t)op->offset + op->total);
+#endif
+#if defined(_WIN32)
+	long processed = -1;
+#else
 	ssize_t processed = -1;
+#endif
 syscall:
 	if (op->direction == DOP_DIR_READ) {
 		if (op->params.type == DISPATCH_IO_STREAM) {
+#if defined(_WIN32)
+			ReadFile((HANDLE)op->fd_entry->fd, buf, (DWORD)len, (LPDWORD)&processed, NULL);
+#else
 			processed = read(op->fd_entry->fd, buf, len);
+#endif
 		} else if (op->params.type == DISPATCH_IO_RANDOM) {
+#if defined(_WIN32)
+			OVERLAPPED ovlOverlapped = {};
+			ovlOverlapped.Offset = off & 0xffffffff;
+			ovlOverlapped.OffsetHigh = (off >> 32) & 0xffffffff;
+			ReadFile((HANDLE)op->fd_entry->fd, buf, (DWORD)len, (LPDWORD)&processed, &ovlOverlapped);
+#else
 			processed = pread(op->fd_entry->fd, buf, len, off);
+#endif
 		}
 	} else if (op->direction == DOP_DIR_WRITE) {
 		if (op->params.type == DISPATCH_IO_STREAM) {
+#if defined(_WIN32)
+			WriteFile((HANDLE)op->fd_entry->fd, buf, (DWORD)len, (LPDWORD)&processed, NULL);
+#else
 			processed = write(op->fd_entry->fd, buf, len);
+#endif
 		} else if (op->params.type == DISPATCH_IO_RANDOM) {
+#if defined(_WIN32)
+			OVERLAPPED ovlOverlapped = {};
+			ovlOverlapped.Offset = off & 0xffffffff;
+			ovlOverlapped.OffsetHigh = (off >> 32) & 0xffffffff;
+			WriteFile((HANDLE)op->fd_entry->fd, buf, (DWORD)len, (LPDWORD)&processed, &ovlOverlapped);
+#else
 			processed = pwrite(op->fd_entry->fd, buf, len, off);
+#endif
 		}
 	}
 	// Encountered an error on the file descriptor
diff --git a/src/io_internal.h b/src/io_internal.h
index 672727fae..d70e07570 100644
--- a/src/io_internal.h
+++ b/src/io_internal.h
@@ -145,7 +145,11 @@ struct dispatch_operation_s {
 	dispatch_queue_t op_q;
 	dispatch_op_direction_t direction; // READ OR WRITE
 	dispatch_io_param_s params;
+#if defined(_WIN32)
+	LONGLONG offset;
+#else
 	off_t offset;
+#endif
 	size_t length;
 	int err;
 	dispatch_io_handler_t handler;
@@ -172,7 +176,11 @@ struct dispatch_io_s {
 	dispatch_fd_entry_t fd_entry;
 	unsigned int atomic_flags;
 	dispatch_fd_t fd, fd_actual;
+#if defined(_WIN32)
+	LONGLONG f_ptr;
+#else
 	off_t f_ptr;
+#endif
 	int err; // contains creation errors only
 };
 
diff --git a/src/object_internal.h b/src/object_internal.h
index 94cb46364..0156503b6 100644
--- a/src/object_internal.h
+++ b/src/object_internal.h
@@ -340,7 +340,7 @@ DISPATCH_ENUM(dispatch_invoke_flags, uint32_t,
 #define _DISPATCH_INVOKE_AUTORELEASE_MASK	  0x00300000u
 );
 
-enum {
+DISPATCH_ENUM(dispatch_object_flags, unsigned long,
 	_DISPATCH_META_TYPE_MASK		= 0xffff0000, // mask for object meta-types
 	_DISPATCH_TYPEFLAGS_MASK		= 0x0000ff00, // mask for object typeflags
 	_DISPATCH_SUB_TYPE_MASK			= 0x000000ff, // mask for object sub-types
@@ -386,8 +386,7 @@ enum {
 
 	DISPATCH_SOURCE_KEVENT_TYPE			= 1 | _DISPATCH_SOURCE_TYPE,
 	DISPATCH_MACH_CHANNEL_TYPE			= 2 | _DISPATCH_SOURCE_TYPE,
-
-};
+);
 
 typedef struct _os_object_vtable_s {
 	_OS_OBJECT_CLASS_HEADER();
diff --git a/src/queue.c b/src/queue.c
index d747a5941..896ed21bc 100644
--- a/src/queue.c
+++ b/src/queue.c
@@ -58,13 +58,13 @@
 #endif
 
 static void _dispatch_sig_thread(void *ctxt);
-static void _dispatch_cache_cleanup(void *value);
+static void DISPATCH_TSD_DTOR_CC _dispatch_cache_cleanup(void *value);
 static void _dispatch_async_f2(dispatch_queue_t dq, dispatch_continuation_t dc);
-static void _dispatch_queue_cleanup(void *ctxt);
-static void _dispatch_wlh_cleanup(void *ctxt);
-static void _dispatch_deferred_items_cleanup(void *ctxt);
-static void _dispatch_frame_cleanup(void *ctxt);
-static void _dispatch_context_cleanup(void *ctxt);
+static void DISPATCH_TSD_DTOR_CC _dispatch_queue_cleanup(void *ctxt);
+static void DISPATCH_TSD_DTOR_CC _dispatch_wlh_cleanup(void *ctxt);
+static void DISPATCH_TSD_DTOR_CC _dispatch_deferred_items_cleanup(void *ctxt);
+static void DISPATCH_TSD_DTOR_CC _dispatch_frame_cleanup(void *ctxt);
+static void DISPATCH_TSD_DTOR_CC _dispatch_context_cleanup(void *ctxt);
 static void _dispatch_queue_barrier_complete(dispatch_queue_t dq,
 		dispatch_qos_t qos, dispatch_wakeup_flags_t flags);
 static void _dispatch_queue_non_barrier_complete(dispatch_queue_t dq);
@@ -87,6 +87,10 @@ static void _dispatch_worker_thread2(int priority, int options, void *context);
 #endif
 #if DISPATCH_USE_PTHREAD_POOL
 static void *_dispatch_worker_thread(void *context);
+#if defined(_WIN32)
+static unsigned WINAPI
+_dispatch_worker_thread_thunk(LPVOID lpParameter);
+#endif
 #endif
 
 #if DISPATCH_COCOA_COMPAT
@@ -101,7 +105,9 @@ static void _dispatch_runloop_queue_handle_dispose(dispatch_queue_t dq);
 #pragma mark dispatch_root_queue
 
 struct dispatch_pthread_root_queue_context_s {
+#if !defined(_WIN32)
 	pthread_attr_t dpq_thread_attr;
+#endif
 	dispatch_block_t dpq_thread_configure;
 	struct dispatch_semaphore_s dpq_thread_mediator;
 	dispatch_pthread_root_queue_observer_hooks_s dpq_observer_hooks;
@@ -763,9 +769,11 @@ _dispatch_root_queue_init_pthread_pool(dispatch_root_queue_context_t qc,
 	qc->dgq_thread_pool_size = thread_pool_size;
 #if DISPATCH_USE_WORKQUEUES
 	if (qc->dgq_qos) {
+#if !defined(_WIN32)
 		(void)dispatch_assume_zero(pthread_attr_init(&pqc->dpq_thread_attr));
 		(void)dispatch_assume_zero(pthread_attr_setdetachstate(
 				&pqc->dpq_thread_attr, PTHREAD_CREATE_DETACHED));
+#endif
 #if HAVE_PTHREAD_WORKQUEUE_QOS
 		(void)dispatch_assume_zero(pthread_attr_set_qos_class_np(
 				&pqc->dpq_thread_attr, qc->dgq_qos, 0));
@@ -906,7 +914,9 @@ libdispatch_init(void)
 #if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
 #include <unistd.h>
 #endif
+#if !defined(_WIN32)
 #include <sys/syscall.h>
+#endif
 
 #ifndef __ANDROID__
 #ifdef SYS_gettid
@@ -923,6 +933,13 @@ gettid(void)
 {
 	return (pid_t)pthread_getthreadid_np();
 }
+#elif defined(_WIN32)
+DISPATCH_ALWAYS_INLINE
+static inline DWORD
+gettid(void)
+{
+	return GetCurrentThreadId();
+}
 #else
 #error "SYS_gettid unavailable on this system"
 #endif /* SYS_gettid */
@@ -944,7 +961,7 @@ _dispatch_install_thread_detach_callback(dispatch_function_t cb)
 }
 #endif
 
-void
+void DISPATCH_TSD_DTOR_CC
 _libdispatch_tsd_cleanup(void *ctx)
 {
 	struct dispatch_tsd *tsd = (struct dispatch_tsd*) ctx;
@@ -980,7 +997,11 @@ DISPATCH_NOINLINE
 void
 libdispatch_tsd_init(void)
 {
+#if defined(_WIN32)
+	FlsSetValue(__dispatch_tsd_key, &__dispatch_tsd);
+#else
 	pthread_setspecific(__dispatch_tsd_key, &__dispatch_tsd);
+#endif /* defined(_WIN32) */
 	__dispatch_tsd.tid = gettid();
 }
 #endif
@@ -1164,8 +1185,6 @@ dispatch_queue_attr_make_with_autorelease_frequency(dispatch_queue_attr_t dqa,
 	case DISPATCH_AUTORELEASE_FREQUENCY_WORK_ITEM:
 	case DISPATCH_AUTORELEASE_FREQUENCY_NEVER:
 		break;
-	default:
-		return DISPATCH_BAD_INPUT;
 	}
 	if (!slowpath(dqa)) {
 		dqa = _dispatch_get_default_queue_attr();
@@ -2035,7 +2054,11 @@ static struct {
 	volatile qos_class_t qos;
 	int default_prio;
 	int policy;
+#if defined(_WIN32)
+	HANDLE hThread;
+#else
 	pthread_t tid;
+#endif
 } _dispatch_mgr_sched;
 
 static dispatch_once_t _dispatch_mgr_sched_pred;
@@ -2053,6 +2076,15 @@ static const int _dispatch_mgr_sched_qos2prio[] = {
 };
 #endif // HAVE_PTHREAD_WORKQUEUE_QOS
 
+#if defined(_WIN32)
+static void
+_dispatch_mgr_sched_init(void *ctx DISPATCH_UNUSED)
+{
+	_dispatch_mgr_sched.policy = 0;
+	_dispatch_mgr_sched.default_prio = THREAD_PRIORITY_NORMAL;
+	_dispatch_mgr_sched.prio = _dispatch_mgr_sched.default_prio;
+}
+#else
 static void
 _dispatch_mgr_sched_init(void *ctxt DISPATCH_UNUSED)
 {
@@ -2080,9 +2112,19 @@ _dispatch_mgr_sched_init(void *ctxt DISPATCH_UNUSED)
 	_dispatch_mgr_sched.default_prio = param.sched_priority;
 	_dispatch_mgr_sched.prio = _dispatch_mgr_sched.default_prio;
 }
+#endif /* defined(_WIN32) */
 #endif // DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES || DISPATCH_USE_KEVENT_WORKQUEUE
 
 #if DISPATCH_USE_MGR_THREAD && DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
+#if defined(_WIN32)
+DISPATCH_NOINLINE
+static PHANDLE
+_dispatch_mgr_root_queue_init(void)
+{
+	dispatch_once_f(&_dispatch_mgr_sched_pred, NULL, _dispatch_mgr_sched_init);
+	return &_dispatch_mgr_sched.hThread;
+}
+#else
 DISPATCH_NOINLINE
 static pthread_t *
 _dispatch_mgr_root_queue_init(void)
@@ -2111,10 +2153,21 @@ _dispatch_mgr_root_queue_init(void)
 	}
 	return &_dispatch_mgr_sched.tid;
 }
+#endif
 
 static inline void
 _dispatch_mgr_priority_apply(void)
 {
+#if defined(_WIN32)
+	int nPriority = _dispatch_mgr_sched.prio;
+	do {
+		if (nPriority > _dispatch_mgr_sched.default_prio) {
+			// TODO(compnerd) set thread scheduling policy
+			dispatch_assume_zero(SetThreadPriority(_dispatch_mgr_sched.hThread, nPriority));
+			nPriority = GetThreadPriority(_dispatch_mgr_sched.hThread);
+		}
+	} while (_dispatch_mgr_sched.prio > nPriority);
+#else
 	struct sched_param param;
 	do {
 		param.sched_priority = _dispatch_mgr_sched.prio;
@@ -2124,12 +2177,19 @@ _dispatch_mgr_priority_apply(void)
 					&param));
 		}
 	} while (_dispatch_mgr_sched.prio > param.sched_priority);
+#endif
 }
 
 DISPATCH_NOINLINE
 void
 _dispatch_mgr_priority_init(void)
 {
+#if defined(_WIN32)
+	int nPriority = GetThreadPriority(_dispatch_mgr_sched.hThread);
+	if (slowpath(_dispatch_mgr_sched.prio > nPriority)) {
+		return _dispatch_mgr_priority_apply();
+	}
+#else
 	struct sched_param param;
 	pthread_attr_t *attr;
 	attr = &_dispatch_mgr_root_queue_pthread_context.dpq_thread_attr;
@@ -2148,9 +2208,11 @@ _dispatch_mgr_priority_init(void)
 	if (slowpath(_dispatch_mgr_sched.prio > param.sched_priority)) {
 		return _dispatch_mgr_priority_apply();
 	}
+#endif
 }
 #endif // DISPATCH_USE_MGR_THREAD && DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
 
+#if !defined(_WIN32)
 #if DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
 DISPATCH_NOINLINE
 static void
@@ -2202,6 +2264,7 @@ _dispatch_mgr_priority_raise(const pthread_attr_t *attr)
 #endif
 }
 #endif // DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
+#endif
 
 #if DISPATCH_USE_KEVENT_WORKQUEUE
 void
@@ -2274,6 +2337,9 @@ _dispatch_pthread_root_queue_create(const char *label, unsigned long flags,
 #endif
 	_dispatch_root_queue_init_pthread_pool(qc, pool_size, true);
 
+#if defined(_WIN32)
+	dispatch_assert(attr == NULL);
+#else
 	if (attr) {
 		memcpy(&pqc->dpq_thread_attr, attr, sizeof(pthread_attr_t));
 		_dispatch_mgr_priority_raise(&pqc->dpq_thread_attr);
@@ -2282,6 +2348,7 @@ _dispatch_pthread_root_queue_create(const char *label, unsigned long flags,
 	}
 	(void)dispatch_assume_zero(pthread_attr_setdetachstate(
 			&pqc->dpq_thread_attr, PTHREAD_CREATE_DETACHED));
+#endif
 	if (configure) {
 		pqc->dpq_thread_configure = _dispatch_Block_copy(configure);
 	}
@@ -2296,6 +2363,9 @@ dispatch_queue_t
 dispatch_pthread_root_queue_create(const char *label, unsigned long flags,
 		const pthread_attr_t *attr, dispatch_block_t configure)
 {
+#if defined(_WIN32)
+	dispatch_assert(attr == NULL);
+#endif
 	return _dispatch_pthread_root_queue_create(label, flags, attr, configure,
 			NULL);
 }
@@ -2345,7 +2415,9 @@ _dispatch_pthread_root_queue_dispose(dispatch_queue_t dq, bool *allow_free)
 	dispatch_root_queue_context_t qc = dq->do_ctxt;
 	dispatch_pthread_root_queue_context_t pqc = qc->dgq_ctxt;
 
+#if !defined(_WIN32)
 	pthread_attr_destroy(&pqc->dpq_thread_attr);
+#endif
 	_dispatch_semaphore_dispose(&pqc->dpq_thread_mediator, NULL);
 	if (pqc->dpq_thread_configure) {
 		Block_release(pqc->dpq_thread_configure);
@@ -2773,7 +2845,7 @@ _dispatch_force_cache_cleanup(void)
 }
 
 DISPATCH_NOINLINE
-static void
+static void DISPATCH_TSD_DTOR_CC
 _dispatch_cache_cleanup(void *value)
 {
 	dispatch_continuation_t dc, next_dc = value;
@@ -4399,7 +4471,6 @@ _dispatch_runloop_queue_set_handle(dispatch_queue_t dq, dispatch_runloop_handle_
 #error "runloop support not implemented on this platform"
 #endif
 }
-#endif // DISPATCH_COCOA_COMPAT
 
 DISPATCH_ALWAYS_INLINE
 static inline dispatch_qos_t
@@ -4410,6 +4481,7 @@ _dispatch_runloop_queue_reset_max_qos(dispatch_queue_class_t dqu)
 	old_state = os_atomic_and_orig2o(dqu._dq, dq_state, ~clear_bits, relaxed);
 	return _dq_state_max_qos(old_state);
 }
+#endif // DISPATCH_COCOA_COMPAT
 
 void
 _dispatch_runloop_queue_wakeup(dispatch_queue_t dq, dispatch_qos_t qos,
@@ -4615,6 +4687,32 @@ _dispatch_global_queue_poke_slow(dispatch_queue_t dq, int n, int floor)
 	} while (!os_atomic_cmpxchgvw2o(qc, dgq_thread_pool_size, t_count,
 			t_count - remaining, &t_count, acquire));
 
+#if defined(_WIN32)
+#if DISPATCH_USE_MGR_THREAD && DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
+	if (slowpath(dq == &_dispatch_mgr_root_queue)) {
+		_dispatch_mgr_root_queue_init();
+	}
+#endif
+	do {
+		_dispatch_retain(dq); // released in _dispatch_worker_thread
+#if DISPATCH_DEBUG
+		unsigned dwStackSize = 0;
+#else
+		unsigned dwStackSize = 64 * 1024;
+#endif
+		uintptr_t hThread = 0;
+		while (!(hThread = _beginthreadex(NULL, dwStackSize, _dispatch_worker_thread_thunk, dq, STACK_SIZE_PARAM_IS_A_RESERVATION, NULL))) {
+			if (errno != EAGAIN) {
+				(void)dispatch_assume(hThread);
+			}
+			_dispatch_temporary_resource_shortage();
+		}
+		if (_dispatch_mgr_sched.prio > _dispatch_mgr_sched.default_prio) {
+			(void)dispatch_assume_zero(SetThreadPriority((HANDLE)hThread, _dispatch_mgr_sched.prio) == TRUE);
+		}
+		CloseHandle((HANDLE)hThread);
+	} while (--remaining);
+#else
 	pthread_attr_t *attr = &pqc->dpq_thread_attr;
 	pthread_t tid, *pthr = &tid;
 #if DISPATCH_USE_MGR_THREAD && DISPATCH_ENABLE_PTHREAD_ROOT_QUEUES
@@ -4631,6 +4729,7 @@ _dispatch_global_queue_poke_slow(dispatch_queue_t dq, int n, int floor)
 			_dispatch_temporary_resource_shortage();
 		}
 	} while (--remaining);
+#endif
 #endif // DISPATCH_USE_PTHREAD_POOL
 }
 
@@ -5951,6 +6050,15 @@ _dispatch_worker_thread2(int priority, int options,
 #if DISPATCH_USE_PTHREAD_POOL
 // 6618342 Contact the team that owns the Instrument DTrace probe before
 //         renaming this symbol
+#if defined(_WIN32)
+static unsigned WINAPI
+_dispatch_worker_thread_thunk(LPVOID lpParameter)
+{
+  _dispatch_worker_thread(lpParameter);
+  return 0;
+}
+#endif
+
 static void *
 _dispatch_worker_thread(void *context)
 {
@@ -5972,7 +6080,9 @@ _dispatch_worker_thread(void *context)
 	}
 
 	// workaround tweaks the kernel workqueue does for us
+#if !defined(_WIN32)
 	_dispatch_sigmask();
+#endif
 	_dispatch_introspection_thread_add();
 
 #if DISPATCH_USE_INTERNAL_WORKQUEUE
@@ -6248,7 +6358,11 @@ dispatch_main(void)
 		pthread_setspecific(dispatch_main_key, &dispatch_main_key);
 		_dispatch_sigmask();
 #endif
+#if defined(_WIN32)
+		_endthreadex(0);
+#else
 		pthread_exit(NULL);
+#endif
 		DISPATCH_INTERNAL_CRASH(errno, "pthread_exit() returned");
 #if HAVE_PTHREAD_MAIN_NP
 	}
@@ -6256,6 +6370,7 @@ dispatch_main(void)
 #endif
 }
 
+#if !defined(_WIN32)
 DISPATCH_NOINLINE DISPATCH_NORETURN
 static void
 _dispatch_sigsuspend(void)
@@ -6266,6 +6381,7 @@ _dispatch_sigsuspend(void)
 		sigsuspend(&mask);
 	}
 }
+#endif
 
 DISPATCH_NORETURN
 static void
@@ -6273,7 +6389,9 @@ _dispatch_sig_thread(void *ctxt DISPATCH_UNUSED)
 {
 	// never returns, so burn bridges behind us
 	_dispatch_clear_stack(0);
+#if !defined(_WIN32)
 	_dispatch_sigsuspend();
+#endif
 }
 
 DISPATCH_NOINLINE
@@ -6320,7 +6438,7 @@ _dispatch_queue_cleanup2(void)
 #endif
 }
 
-static void
+static void DISPATCH_TSD_DTOR_CC
 _dispatch_queue_cleanup(void *ctxt)
 {
 	if (ctxt == &_dispatch_main_q) {
@@ -6331,7 +6449,7 @@ _dispatch_queue_cleanup(void *ctxt)
 			"Premature thread exit while a dispatch queue is running");
 }
 
-static void
+static void DISPATCH_TSD_DTOR_CC
 _dispatch_wlh_cleanup(void *ctxt)
 {
 	// POSIX defines that destructors are only called if 'ctxt' is non-null
@@ -6341,7 +6459,7 @@ _dispatch_wlh_cleanup(void *ctxt)
 }
 
 DISPATCH_NORETURN
-static void
+static void DISPATCH_TSD_DTOR_CC
 _dispatch_deferred_items_cleanup(void *ctxt)
 {
 	// POSIX defines that destructors are only called if 'ctxt' is non-null
@@ -6350,7 +6468,7 @@ _dispatch_deferred_items_cleanup(void *ctxt)
 }
 
 DISPATCH_NORETURN
-static void
+static DISPATCH_TSD_DTOR_CC void
 _dispatch_frame_cleanup(void *ctxt)
 {
 	// POSIX defines that destructors are only called if 'ctxt' is non-null
@@ -6359,7 +6477,7 @@ _dispatch_frame_cleanup(void *ctxt)
 }
 
 DISPATCH_NORETURN
-static void
+static void DISPATCH_TSD_DTOR_CC
 _dispatch_context_cleanup(void *ctxt)
 {
 	// POSIX defines that destructors are only called if 'ctxt' is non-null
diff --git a/src/shims.h b/src/shims.h
index 4a78bfc03..278b6ce5d 100644
--- a/src/shims.h
+++ b/src/shims.h
@@ -27,8 +27,12 @@
 #ifndef __DISPATCH_OS_SHIMS__
 #define __DISPATCH_OS_SHIMS__
 
+#if !defined(_WIN32)
 #include <pthread.h>
-#if defined(__linux__) || defined(__FreeBSD__)
+#endif
+#if defined(_WIN32)
+#include "shims/generic_win_stubs.h"
+#elif defined(__unix__)
 #include "shims/generic_unix_stubs.h"
 #endif
 
@@ -74,12 +78,6 @@ size_t strlcpy(char *dst, const char *src, size_t size);
 
 #endif // HAVE_STRLCPY
 
-
-#if TARGET_OS_WIN32
-#define bzero(ptr,len) memset((ptr), 0, (len))
-#define snprintf _snprintf
-#endif // TARGET_OS_WIN32
-
 #if PTHREAD_WORKQUEUE_SPI_VERSION < 20140716
 static inline int
 _pthread_workqueue_override_start_direct(mach_port_t thread,
diff --git a/src/shims/atomic.h b/src/shims/atomic.h
index 60f2891f8..1cb094082 100644
--- a/src/shims/atomic.h
+++ b/src/shims/atomic.h
@@ -32,7 +32,7 @@
 #endif
 
 // FreeBSD only defines _Bool in C mode. In C++ mode _Bool is not being defined.
-#if defined(__cplusplus) && defined(__FreeBSD__)
+#if defined(__cplusplus) && (defined(__FreeBSD__) || defined(_WIN32))
 #define _Bool bool
 #endif
 #include <stdatomic.h>
diff --git a/src/shims/generic_win_stubs.c b/src/shims/generic_win_stubs.c
new file mode 100644
index 000000000..f6984a2d8
--- /dev/null
+++ b/src/shims/generic_win_stubs.c
@@ -0,0 +1,4 @@
+
+/*
+ * Stubbed out static data
+ */
diff --git a/src/shims/generic_win_stubs.h b/src/shims/generic_win_stubs.h
new file mode 100644
index 000000000..d7a6f214c
--- /dev/null
+++ b/src/shims/generic_win_stubs.h
@@ -0,0 +1,50 @@
+
+#ifndef __DISPATCH__STUBS__INTERNAL
+#define __DISPATCH__STUBS__INTERNAL
+
+#include <stdint.h>
+
+#include <Windows.h>
+
+#include <io.h>
+#include <process.h>
+
+/*
+ * Stub out defines for some mach types and related macros
+ */
+
+typedef uint32_t mach_port_t;
+
+#define MACH_PORT_NULL (0)
+
+typedef uint32_t mach_msg_bits_t;
+typedef void *mach_msg_header_t;
+
+/*
+ * Stub out defines for other missing types
+ */
+
+// SIZE_T_MAX should not be hardcoded like this here.
+#ifndef SIZE_T_MAX
+#define SIZE_T_MAX (~(size_t)0)
+#endif
+
+typedef __typeof__(_Generic((__SIZE_TYPE__)0,                                  \
+			    unsigned long long int : (long long int)0,         \
+			    unsigned long int : (long int)0,                   \
+			    unsigned int : (int)0,                             \
+			    unsigned short : (short)0,                         \
+			    unsigned char : (signed char)0)) ssize_t;
+
+#define S_ISDIR(mode)  (((mode) & S_IFMT) == S_IFDIR)
+#define S_ISFIFO(mode) ((mode) & _S_IFIFO)
+#define S_ISREG(mode)  ((mode) & _S_IFREG)
+#define S_ISSOCK(mode) 0
+
+#define O_NONBLOCK 04000
+
+#define bzero(ptr,len) memset((ptr), 0, (len))
+#define snprintf _snprintf
+
+#endif
+
diff --git a/src/shims/hw_config.h b/src/shims/hw_config.h
index 485dad663..e78872761 100644
--- a/src/shims/hw_config.h
+++ b/src/shims/hw_config.h
@@ -43,8 +43,6 @@
 #error "could not determine pointer size as a constant int"
 #endif // __SIZEOF_POINTER__
 
-#if !TARGET_OS_WIN32
-
 typedef enum {
 	_dispatch_hw_config_logical_cpus,
 	_dispatch_hw_config_physical_cpus,
@@ -115,6 +113,64 @@ _dispatch_hw_get_config(_dispatch_hw_config_t c)
 			return (uint32_t)sysconf(_SC_NPROCESSORS_ONLN);
 		}
 	}
+#elif defined(_WIN32)
+	PSYSTEM_LOGICAL_PROCESSOR_INFORMATION slpiInfo = NULL;
+	PSYSTEM_LOGICAL_PROCESSOR_INFORMATION slpiCurrent = NULL;
+	DWORD dwProcessorLogicalCount = 0;
+	DWORD dwProcessorPackageCount = 0;
+	DWORD dwProcessorCoreCount = 0;
+	DWORD dwSize = 0;
+
+	while (true) {
+		DWORD dwResult;
+
+		if (GetLogicalProcessorInformation(slpiInfo, &dwSize))
+			break;
+
+		dwResult = GetLastError();
+
+		if (slpiInfo)
+			free(slpiInfo);
+
+		if (dwResult == ERROR_INSUFFICIENT_BUFFER) {
+			slpiInfo = (PSYSTEM_LOGICAL_PROCESSOR_INFORMATION)malloc(dwSize);
+			dispatch_assert(slpiInfo);
+		} else {
+			slpiInfo = NULL;
+			dwSize = 0;
+			break;
+		}
+	}
+
+	for (slpiCurrent = slpiInfo;
+	     dwSize >= sizeof(SYSTEM_LOGICAL_PROCESSOR_INFORMATION);
+	     slpiCurrent++, dwSize -= sizeof(SYSTEM_LOGICAL_PROCESSOR_INFORMATION)) {
+		switch (slpiCurrent->Relationship) {
+		case RelationProcessorCore:
+			++dwProcessorCoreCount;
+			dwProcessorLogicalCount += __popcnt64(slpiCurrent->ProcessorMask);
+			break;
+		case RelationProcessorPackage:
+			++dwProcessorPackageCount;
+			break;
+		case RelationNumaNode:
+		case RelationCache:
+		case RelationGroup:
+		case RelationAll:
+			break;
+		}
+	}
+
+	free(slpiInfo);
+
+	switch (c) {
+	case _dispatch_hw_config_logical_cpus:
+		return dwProcessorLogicalCount;
+	case _dispatch_hw_config_physical_cpus:
+		return dwProcessorPackageCount;
+	case _dispatch_hw_config_active_cpus:
+		return dwProcessorCoreCount;
+	}
 #else
 	const char *name = NULL;
 	int r;
@@ -160,31 +216,4 @@ _dispatch_hw_config_init(void)
 
 #endif // DISPATCH_HAVE_HW_CONFIG_COMMPAGE
 
-#else // TARGET_OS_WIN32
-
-static inline long
-_dispatch_count_bits(unsigned long value)
-{
-	long bits = 0;
-	while (value) {
-		bits += (value & 1);
-		value = value >> 1;
-	}
-	return bits;
-}
-
-static inline uint32_t
-_dispatch_get_ncpus(void)
-{
-	uint32_t val;
-	DWORD_PTR procmask, sysmask;
-	if (GetProcessAffinityMask(GetCurrentProcess(), &procmask, &sysmask)) {
-		val = _dispatch_count_bits(procmask);
-	} else {
-		val = 1;
-	}
-	return val;
-}
-#endif // TARGET_OS_WIN32
-
 #endif /* __DISPATCH_SHIMS_HW_CONFIG__ */
diff --git a/src/shims/lock.c b/src/shims/lock.c
index bc5545070..27b0c10a0 100644
--- a/src/shims/lock.c
+++ b/src/shims/lock.c
@@ -51,6 +51,21 @@ _dispatch_thread_switch(dispatch_lock value, dispatch_lock_options_t flags,
 #endif // HAVE_UL_UNFAIR_LOCK
 #endif
 
+#if defined(_WIN32)
+#if !HAVE_UL_UNFAIR_LOCK
+DISPATCH_ALWAYS_INLINE
+static inline void
+_dispatch_thread_switch(dispatch_lock value, dispatch_lock_options_t flags,
+		uint32_t timeout)
+{
+	(void)value;
+	(void)flags;
+	(void)timeout;
+	SwitchToThread();
+}
+#endif
+#endif
+
 #pragma mark - semaphores
 
 #if USE_MACH_SEM
@@ -230,9 +245,7 @@ _push_timer_resolution(DWORD ms)
 	// aim for the best resolution we can accomplish
 	dispatch_once(&once, ^{
 		TIMECAPS tc;
-		MMRESULT res;
-		res = timeGetDevCaps(&tc, sizeof(tc));
-		if (res == MMSYSERR_NOERROR) {
+		if (timeGetDevCaps(&tc, sizeof(tc)) == MMSYSERR_NOERROR) {
 			best_resolution = min(max(tc.wPeriodMin, best_resolution),
 					tc.wPeriodMax);
 		}
@@ -302,7 +315,7 @@ _dispatch_sema4_timedwait(_dispatch_sema4_t *sema, dispatch_time_t timeout)
 	nsec = _dispatch_timeout(timeout);
 	msec = (DWORD)(nsec / (uint64_t)1000000);
 	resolution = _push_timer_resolution(msec);
-	wait_result = WaitForSingleObject(dsema->dsema_handle, msec);
+	wait_result = WaitForSingleObject(sema, msec);
 	_pop_timer_resolution(resolution);
 	return wait_result == WAIT_TIMEOUT;
 }
@@ -440,6 +453,8 @@ _dispatch_wait_on_address(uint32_t volatile *address, uint32_t value,
 	_dispatch_ulock_wait((uint32_t *)address, value, 0, flags);
 #elif HAVE_FUTEX
 	_dispatch_futex_wait((uint32_t *)address, value, NULL, FUTEX_PRIVATE_FLAG);
+#elif defined(_WIN32)
+	WaitOnAddress(address, (PVOID)(uintptr_t)value, sizeof(value), INFINITE);
 #else
 	mach_msg_timeout_t timeout = 1;
 	while (os_atomic_load(address, relaxed) == value) {
@@ -456,6 +471,8 @@ _dispatch_wake_by_address(uint32_t volatile *address)
 	_dispatch_ulock_wake((uint32_t *)address, ULF_WAKE_ALL);
 #elif HAVE_FUTEX
 	_dispatch_futex_wake((uint32_t *)address, INT_MAX, FUTEX_PRIVATE_FLAG);
+#elif defined(_WIN32)
+	WakeByAddressAll((uint32_t *)address);
 #else
 	(void)address;
 #endif
@@ -545,11 +562,11 @@ void
 _dispatch_unfair_lock_lock_slow(dispatch_unfair_lock_t dul,
 		dispatch_lock_options_t flags)
 {
-	dispatch_lock cur, value_self = _dispatch_lock_value_for_self();
+	dispatch_lock cur, self = _dispatch_lock_value_for_self();
 	uint32_t timeout = 1;
 
 	while (unlikely(!os_atomic_cmpxchgv(&dul->dul_lock,
-			DLOCK_OWNER_NULL, value_self, &cur, acquire))) {
+			DLOCK_OWNER_NULL, self, &cur, acquire))) {
 		if (unlikely(_dispatch_lock_is_locked_by(cur, self))) {
 			DISPATCH_CLIENT_CRASH(0, "trying to lock recursively");
 		}
diff --git a/src/shims/lock.h b/src/shims/lock.h
index 37a3ecfc8..4a9bd7836 100644
--- a/src/shims/lock.h
+++ b/src/shims/lock.h
@@ -79,6 +79,27 @@ _dispatch_lock_owner(dispatch_lock lock_value)
 	return lock_value & DLOCK_OWNER_MASK;
 }
 
+#elif defined(_WIN32)
+
+#include <Windows.h>
+
+typedef DWORD dispatch_tid;
+typedef uint32_t dispatch_lock;
+
+#define DLOCK_OWNER_NULL			((dispatch_tid)0)
+#define DLOCK_OWNER_MASK			((dispatch_lock)0xfffffffc)
+#define DLOCK_WAITERS_BIT			((dispatch_lock)0x00000001)
+#define DLOCK_FAILED_TRYLOCK_BIT		((dispatch_lock)0x00000002)
+
+#define _dispatch_tid_self()		((dispatch_tid)(_dispatch_get_tsd_base()->tid << 2))
+
+DISPATCH_ALWAYS_INLINE
+static inline dispatch_tid
+_dispatch_lock_owner(dispatch_lock lock_value)
+{
+	return lock_value & DLOCK_OWNER_MASK;
+}
+
 #else
 #  error define _dispatch_lock encoding scheme for your platform here
 #endif
diff --git a/src/shims/time.h b/src/shims/time.h
index 6cc5a1603..2e2760574 100644
--- a/src/shims/time.h
+++ b/src/shims/time.h
@@ -31,7 +31,7 @@
 #error "Please #include <dispatch/dispatch.h> instead of this file directly."
 #endif
 
-#if TARGET_OS_WIN32
+#if defined(_WIN32)
 static inline unsigned int
 sleep(unsigned int seconds)
 {
@@ -106,7 +106,7 @@ _dispatch_get_nanoseconds(void)
 	struct timespec ts;
 	dispatch_assume_zero(clock_gettime(CLOCK_REALTIME, &ts));
 	return _dispatch_timespec_to_nano(ts);
-#elif TARGET_OS_WIN32
+#elif defined(_WIN32)
 	// FILETIME is 100-nanosecond intervals since January 1, 1601 (UTC).
 	FILETIME ft;
 	ULARGE_INTEGER li;
@@ -147,9 +147,12 @@ _dispatch_absolute_time(void)
 	struct timespec ts;
 	dispatch_assume_zero(clock_gettime(CLOCK_MONOTONIC, &ts));
 	return _dispatch_timespec_to_nano(ts);
-#elif TARGET_OS_WIN32
-	LARGE_INTEGER now;
-	return QueryPerformanceCounter(&now) ? now.QuadPart : 0;
+#elif defined(_WIN32)
+	ULONGLONG ullTime;
+	if (!QueryUnbiasedInterruptTime(&ullTime))
+		return 0;
+
+	return ullTime * 100ull;
 #else
 #error platform needs to implement _dispatch_absolute_time()
 #endif
diff --git a/src/shims/tsd.h b/src/shims/tsd.h
index c119e4f01..7271f45c9 100644
--- a/src/shims/tsd.h
+++ b/src/shims/tsd.h
@@ -69,6 +69,7 @@ typedef struct { void *a; void *b; } dispatch_tsd_pair_t;
 #define __TSD_MACH_SPECIAL_REPLY 8
 #endif
 
+
 static const unsigned long dispatch_priority_key	= __TSD_THREAD_QOS_CLASS;
 static const unsigned long dispatch_r2k_key			= __TSD_RETURN_TO_KERNEL;
 
@@ -99,15 +100,44 @@ _dispatch_thread_key_create(const unsigned long *k, void (*d)(void *))
 }
 #elif DISPATCH_USE_THREAD_LOCAL_STORAGE
 
+#if defined(_WIN32)
+#define DISPATCH_TSD_DTOR_CC __stdcall
+#else
+#define DISPATCH_TSD_DTOR_CC
+#endif
+
+#if defined(_WIN32)
+
 DISPATCH_TSD_INLINE
 static inline void
-_dispatch_thread_key_create(pthread_key_t *k, void (*d)(void *))
+_dispatch_thread_key_create(DWORD *k, void (DISPATCH_TSD_DTOR_CC *d)(void *))
+{
+	dispatch_assert_zero((*k = FlsAlloc(d)));
+}
+
+extern __declspec(thread) struct dispatch_tsd __dispatch_tsd;
+extern DWORD __dispatch_tsd_key;
+
+#else
+
+DISPATCH_TSD_INLINE
+static inline void
+_dispatch_thread_key_create(pthread_key_t *k, void (DISPATCH_TSD_DTOR_CC *d)(void *))
 {
 	dispatch_assert_zero(pthread_key_create(k, d));
 }
 
+extern __thread struct dispatch_tsd __dispatch_tsd;
+extern pthread_key_t __dispatch_tsd_key;
+
+#endif
+
 struct dispatch_tsd {
+#if defined(_WIN32)
+	DWORD tid;
+#else
 	pid_t tid;
+#endif
 	void *dispatch_queue_key;
 	void *dispatch_frame_key;
 	void *dispatch_cache_key;
@@ -126,8 +156,6 @@ struct dispatch_tsd {
 	void *dispatch_deferred_items_key;
 };
 
-extern __thread struct dispatch_tsd __dispatch_tsd;
-extern pthread_key_t __dispatch_tsd_key;
 extern void libdispatch_tsd_init(void);
 extern void _libdispatch_tsd_cleanup(void *ctx);
 
@@ -285,7 +313,7 @@ _dispatch_thread_setspecific_packed_pair(pthread_key_t k1, pthread_key_t k2,
 }
 #endif
 
-#if TARGET_OS_WIN32
+#if defined(_WIN32)
 #define _dispatch_thread_self() ((uintptr_t)GetCurrentThreadId())
 #else
 #if DISPATCH_USE_DIRECT_TSD
@@ -296,7 +324,7 @@ _dispatch_thread_setspecific_packed_pair(pthread_key_t k1, pthread_key_t k2,
 #endif
 #endif
 
-#if TARGET_OS_WIN32
+#if defined(_WIN32)
 #define _dispatch_thread_port() ((mach_port_t)0)
 #elif !DISPATCH_USE_THREAD_LOCAL_STORAGE
 #if DISPATCH_USE_DIRECT_TSD
diff --git a/src/shims/yield.h b/src/shims/yield.h
index 99864af40..2373e5075 100644
--- a/src/shims/yield.h
+++ b/src/shims/yield.h
@@ -83,6 +83,11 @@
 #define _dispatch_contention_spins() \
 		((DISPATCH_CONTENTION_SPINS_MIN) + ((DISPATCH_CONTENTION_SPINS_MAX) - \
 		(DISPATCH_CONTENTION_SPINS_MIN)) / 2)
+#elif defined(_WIN32)
+#define _dispatch_contention_spins() ({                                        \
+		unsigned int _value;                                           \
+		rand_s(&_value);                                               \
+		(_value & DISPATCH_CONTENTION_SPINS_MAX) | DISPATCH_CONTENTION_SPINS_MIN; })
 #else
 // Use randomness to prevent threads from resonating at the same
 // frequency and permanently contending. All threads sharing the same
@@ -151,7 +156,24 @@
 		SWITCH_OPTION_WAIT, (((u)-1)/1000)+1)
 #endif
 #else
+#if defined(_WIN32)
+DISPATCH_INLINE void
+_dispatch_contention_usleep(uint64_t useconds) {
+	static BOOL bQPFExecuted = FALSE;
+	static LARGE_INTEGER liFreq;
+	LARGE_INTEGER liStart, liNow;
+
+	if (!bQPFExecuted)
+		bQPFExecuted = QueryPerformanceFrequency(&liFreq);
+
+	QueryPerformanceCounter(&liStart);
+	do {
+		QueryPerformanceCounter(&liNow);
+	} while ((liNow.QuadPart - liStart.QuadPart) / (float)liFreq.QuadPart * 1000 * 1000 < useconds);
+}
+#else
 #define _dispatch_contention_usleep(u) usleep((u))
+#endif
 #endif // HAVE_MACH
 
 #endif // __DISPATCH_SHIMS_YIELD__
diff --git a/src/transform.c b/src/transform.c
index 44a1271e4..7f2c55677 100644
--- a/src/transform.c
+++ b/src/transform.c
@@ -30,6 +30,9 @@
 #include <sys/endian.h>
 #define OSLittleEndian _LITTLE_ENDIAN
 #define OSBigEndian _BIG_ENDIAN
+#elif defined(_WIN32)
+#define OSLittleEndian 1234
+#define OSBigEndian 4321
 #endif
 
 #if defined(__linux__) || defined(__FreeBSD__)
@@ -37,6 +40,11 @@
 #define OSSwapBigToHostInt16 be16toh
 #define OSSwapHostToLittleInt16 htole16
 #define OSSwapHostToBigInt16 htobe16
+#elif defined(_WIN32)
+#define OSSwapLittleToHostInt16
+#define OSSwapBigToHostInt16 ntohs
+#define OSSwapHostToLittleInt16
+#define OSSwapHostToBigInt16 htons
 #endif
 
 #if defined(__LITTLE_ENDIAN__)
diff --git a/src/voucher.c b/src/voucher.c
index 458e2f0a4..e32bd29dc 100644
--- a/src/voucher.c
+++ b/src/voucher.c
@@ -157,7 +157,7 @@ voucher_release(voucher_t voucher)
 	return _voucher_release(voucher);
 }
 
-void
+void DISPATCH_TSD_DTOR_CC
 _voucher_thread_cleanup(void *voucher)
 {
 	// when a thread exits and has a voucher left, the kernel
@@ -1515,7 +1515,7 @@ voucher_decrement_importance_count4CF(voucher_t v)
 	(void)v;
 }
 
-void
+void DISPATCH_TSD_DTOR_CC
 _voucher_thread_cleanup(void *voucher)
 {
 	(void)voucher;
diff --git a/src/voucher_internal.h b/src/voucher_internal.h
index 772c8c434..9f5d72bc5 100644
--- a/src/voucher_internal.h
+++ b/src/voucher_internal.h
@@ -97,7 +97,7 @@ void _voucher_activity_swap(firehose_activity_id_t old_id,
 void _voucher_xref_dispose(voucher_t voucher);
 void _voucher_dispose(voucher_t voucher);
 size_t _voucher_debug(voucher_t v, char* buf, size_t bufsiz);
-void _voucher_thread_cleanup(void *voucher);
+void DISPATCH_TSD_DTOR_CC _voucher_thread_cleanup(void *voucher);
 mach_voucher_t _voucher_get_mach_voucher(voucher_t voucher);
 voucher_t _voucher_create_without_importance(voucher_t voucher);
 voucher_t _voucher_create_accounting_voucher(voucher_t voucher);