diff --git a/Sources/SwiftSyntaxMacros/CMakeLists.txt b/Sources/SwiftSyntaxMacros/CMakeLists.txt
index f8fdc38f639..f95e55fa732 100644
--- a/Sources/SwiftSyntaxMacros/CMakeLists.txt
+++ b/Sources/SwiftSyntaxMacros/CMakeLists.txt
@@ -21,7 +21,9 @@ add_swift_host_library(SwiftSyntaxMacros
 
   AbstractSourceLocation.swift
   BasicMacroExpansionContext.swift
+  FunctionParameterUtils.swift
   MacroExpansionContext.swift
+  MacroReplacement.swift
   MacroSystem.swift
   Syntax+MacroEvaluation.swift
 )
diff --git a/Sources/SwiftSyntaxMacros/FunctionParameterUtils.swift b/Sources/SwiftSyntaxMacros/FunctionParameterUtils.swift
new file mode 100644
index 00000000000..9ad940b3ff0
--- /dev/null
+++ b/Sources/SwiftSyntaxMacros/FunctionParameterUtils.swift
@@ -0,0 +1,32 @@
+import SwiftSyntax
+
+extension FunctionParameterSyntax {
+  /// Retrieve the name of the parameter as it is used in source.
+  ///
+  /// Example:
+  ///
+  ///     func f(a: Int, _ b: Int, c see: Int) { ... }
+  ///
+  /// The parameter names for these three parameters are `a`, `b`, and `see`,
+  /// respectively.
+  var parameterName: TokenSyntax? {
+    // If there were two names, the second is the parameter name.
+    if let secondName = secondName {
+      if secondName.text == "_" {
+        return nil
+      }
+
+      return secondName
+    }
+
+    if let firstName = firstName {
+      if firstName.text == "_" {
+        return nil
+      }
+
+      return firstName
+    }
+
+    return nil
+  }
+}
diff --git a/Sources/SwiftSyntaxMacros/MacroReplacement.swift b/Sources/SwiftSyntaxMacros/MacroReplacement.swift
new file mode 100644
index 00000000000..edbef9a1288
--- /dev/null
+++ b/Sources/SwiftSyntaxMacros/MacroReplacement.swift
@@ -0,0 +1,312 @@
+import SwiftDiagnostics
+import SwiftSyntax
+import SwiftSyntaxBuilder
+
+enum MacroExpanderError: DiagnosticMessage {
+  case undefined
+  case definitionNotMacroExpansion
+  case nonParameterReference(TokenSyntax)
+  case nonLiteralOrParameter(ExprSyntax)
+
+  var message: String {
+    switch self {
+    case .undefined:
+      return "macro expansion requires a definition"
+
+    case .definitionNotMacroExpansion:
+      return "macro definition must itself by a macro expansion expression (starting with '#')"
+
+    case .nonParameterReference(let name):
+      return "reference to value '\(name.text)' that is not a macro parameter in expansion"
+
+    case .nonLiteralOrParameter:
+      return "only literals and macro parameters are permitted in expansion"
+    }
+  }
+
+  var diagnosticID: MessageID {
+    .init(domain: "SwiftMacros", id: "\(self)")
+  }
+
+  var severity: DiagnosticSeverity {
+    .error
+  }
+}
+
+/// Provide the definition of a macro
+public enum MacroDefinition {
+  /// An externally-defined macro, known by its type name and the module in
+  /// which that type resides, which uses the deprecated syntax `A.B`.
+  case deprecatedExternal(node: Syntax, module: String, type: String)
+
+  /// A macro that is defined by expansion of another macro.
+  ///
+  /// The definition has the macro expansion expression itself, along with
+  /// sequence of replacements for subtrees that refer to parameters of the
+  /// defining macro. These subtrees will need to be replaced with the text of
+  /// the corresponding argument to the macro, which can be accomplished with
+  /// `MacroDeclSyntax.expandDefinition`.
+  case expansion(MacroExpansionExprSyntax, replacements: [Replacement])
+}
+
+extension MacroDefinition {
+  /// A replacement that occurs as part of an expanded macro definition.
+  public struct Replacement {
+    /// A reference to a parameter as it occurs in the macro expansion expression.
+    public let reference: IdentifierExprSyntax
+
+    /// The index of the parameter in the defining macro.
+    public let parameterIndex: Int
+  }
+}
+
+fileprivate class ParameterReplacementVisitor: SyntaxAnyVisitor {
+  let macro: MacroDeclSyntax
+  var replacements: [MacroDefinition.Replacement] = []
+  var diagnostics: [Diagnostic] = []
+
+  init(macro: MacroDeclSyntax) {
+    self.macro = macro
+    super.init(viewMode: .fixedUp)
+  }
+
+  // Integer literals
+  override func visit(_ node: IntegerLiteralExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // Floating point literals
+  override func visit(_ node: FloatLiteralExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // nil literals
+  override func visit(_ node: NilLiteralExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // String literals
+  override func visit(_ node: StringLiteralExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // Array literals
+  override func visit(_ node: ArrayExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // Dictionary literals
+  override func visit(_ node: DictionaryExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // Tuple literals
+  override func visit(_ node: TupleExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // Macro uses.
+  override func visit(_ node: MacroExpansionExprSyntax) -> SyntaxVisitorContinueKind {
+    .visitChildren
+  }
+
+  // References to declarations. Only accept those that refer to a parameter
+  // of a macro.
+  override func visit(_ node: IdentifierExprSyntax) -> SyntaxVisitorContinueKind {
+    let identifier = node.identifier
+
+    // FIXME: This will go away.
+    guard case let .functionLike(signature) = macro.signature else {
+      return .visitChildren
+    }
+
+    let matchedParameter = signature.input.parameterList.enumerated().first { (index, parameter) in
+      if identifier.text == "_" {
+        return false
+      }
+
+      guard let parameterName = parameter.parameterName else {
+        return false
+      }
+
+      return identifier.text == parameterName.text
+    }
+
+    guard let (parameterIndex, _) = matchedParameter else {
+      // We have a reference to something that isn't a parameter of the macro.
+      diagnostics.append(
+        Diagnostic(
+          node: Syntax(identifier),
+          message: MacroExpanderError.nonParameterReference(identifier)
+        )
+      )
+
+      return .visitChildren
+    }
+
+    replacements.append(.init(reference: node, parameterIndex: parameterIndex))
+    return .visitChildren
+  }
+
+  override func visitAny(_ node: Syntax) -> SyntaxVisitorContinueKind {
+    if let expr = node.as(ExprSyntax.self) {
+      // We have an expression that is not one of the allowed forms, so
+      // diagnose it.
+      diagnostics.append(
+        Diagnostic(
+          node: node,
+          message: MacroExpanderError.nonLiteralOrParameter(expr)
+        )
+      )
+
+      return .skipChildren
+    }
+
+    return .visitChildren
+  }
+
+}
+
+extension MacroDeclSyntax {
+  /// Check the definition of the given macro.
+  ///
+  /// Macros are defined by an expression, which must itself be a macro
+  /// expansion. Check the definition and produce a semantic representation of
+  /// it  or one of the "builtin"
+  ///
+  /// Compute the sequence of parameter replacements required when expanding
+  /// the definition of a non-external macro.
+  ///
+  /// If there are an errors that prevent expansion, the diagnostics will be
+  /// wrapped into a  an error that prevents expansion, that error is thrown.
+  public func checkDefinition() throws -> MacroDefinition {
+    // Cannot compute replacements for an undefined macro.
+    guard let originalDefinition = definition?.value else {
+      let undefinedDiag = Diagnostic(
+        node: Syntax(self),
+        message: MacroExpanderError.undefined
+      )
+
+      throw DiagnosticsError(diagnostics: [undefinedDiag])
+    }
+
+    /// Recognize the deprecated syntax A.B. Clients will need to
+    /// handle this themselves.
+    if let memberAccess = originalDefinition.as(MemberAccessExprSyntax.self),
+      let base = memberAccess.base,
+      let baseName = base.as(IdentifierExprSyntax.self)?.identifier
+    {
+      let memberName = memberAccess.name
+      return .deprecatedExternal(
+        node: Syntax(memberAccess),
+        module: baseName.trimmedDescription,
+        type: memberName.trimmedDescription
+      )
+    }
+
+    // Make sure we have a macro expansion expression.
+    guard let definition = originalDefinition.as(MacroExpansionExprSyntax.self) else {
+      let badDefinitionDiag =
+        Diagnostic(
+          node: Syntax(originalDefinition),
+          message: MacroExpanderError.definitionNotMacroExpansion
+        )
+
+      throw DiagnosticsError(diagnostics: [badDefinitionDiag])
+    }
+
+    let visitor = ParameterReplacementVisitor(macro: self)
+    visitor.walk(definition)
+
+    if !visitor.diagnostics.isEmpty {
+      throw DiagnosticsError(diagnostics: visitor.diagnostics)
+    }
+
+    return .expansion(definition, replacements: visitor.replacements)
+  }
+}
+
+/// Syntax rewrite that performs macro expansion by textually replacing
+/// uses of macro parameters with their corresponding arguments.
+private final class MacroExpansionRewriter: SyntaxRewriter {
+  let parameterReplacements: [IdentifierExprSyntax: Int]
+  let arguments: [ExprSyntax]
+
+  init(parameterReplacements: [IdentifierExprSyntax: Int], arguments: [ExprSyntax]) {
+    self.parameterReplacements = parameterReplacements
+    self.arguments = arguments
+  }
+
+  override func visit(_ node: IdentifierExprSyntax) -> ExprSyntax {
+    guard let parameterIndex = parameterReplacements[node] else {
+      return super.visit(node)
+    }
+
+    // Swap in the argument for this parameter
+    return arguments[parameterIndex].trimmed
+  }
+}
+
+extension MacroDeclSyntax {
+  /// Expand the definition of this macro when provided with the given
+  /// argument list.
+  private func expand(
+    argumentList: TupleExprElementListSyntax?,
+    definition: MacroExpansionExprSyntax,
+    replacements: [MacroDefinition.Replacement]
+  ) -> ExprSyntax {
+    // FIXME: Do real call-argument matching between the argument list and the
+    // macro parameter list, porting over from the compiler.
+    let arguments: [ExprSyntax] =
+      argumentList?.map { element in
+        element.expression
+      } ?? []
+
+    return MacroExpansionRewriter(
+      parameterReplacements: Dictionary(
+        uniqueKeysWithValues: replacements.map { replacement in
+          (replacement.reference, replacement.parameterIndex)
+        }
+      ),
+      arguments: arguments
+    ).visit(definition)
+  }
+
+  /// Given a freestanding macro expansion syntax node that references this
+  /// macro declaration, expand the macro by substituting the arguments from
+  /// the macro expansion into the parameters that are used in the definition.
+  public func expand<Node: FreestandingMacroExpansionSyntax>(
+    _ node: Node,
+    definition: MacroExpansionExprSyntax,
+    replacements: [MacroDefinition.Replacement]
+  ) -> ExprSyntax {
+    return expand(
+      argumentList: node.argumentList,
+      definition: definition,
+      replacements: replacements
+    )
+  }
+
+  /// Given an attached macro expansion syntax node that references this
+  /// macro declaration, expand the macro by substituting the arguments from
+  /// the expansion into the parameters that are used in the definition.
+  public func expand(
+    _ node: AttributeSyntax,
+    definition: MacroExpansionExprSyntax,
+    replacements: [MacroDefinition.Replacement]
+  ) -> ExprSyntax {
+    // Dig out the argument list.
+    let argumentList: TupleExprElementListSyntax?
+    if case let .argumentList(argList) = node.argument {
+      argumentList = argList
+    } else {
+      argumentList = nil
+    }
+
+    return expand(
+      argumentList: argumentList,
+      definition: definition,
+      replacements: replacements
+    )
+  }
+}
diff --git a/Tests/SwiftSyntaxMacrosTest/MacroReplacementTests.swift b/Tests/SwiftSyntaxMacrosTest/MacroReplacementTests.swift
new file mode 100644
index 00000000000..3ddf8a9d153
--- /dev/null
+++ b/Tests/SwiftSyntaxMacrosTest/MacroReplacementTests.swift
@@ -0,0 +1,117 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2023 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+import SwiftDiagnostics
+import SwiftParser
+import SwiftSyntax
+import SwiftSyntaxBuilder
+import SwiftSyntaxMacros
+import _SwiftSyntaxTestSupport
+import XCTest
+
+final class MacroReplacementTests: XCTestCase {
+  func testMacroDefinitionGood() throws {
+    let macro: DeclSyntax =
+      """
+      macro expand1(a: Int, b: Int) = #otherMacro(first: b, second: ["a": a], third: [3.14159, 2.71828], fourth: 4)
+      """
+
+    let definition = try macro.as(MacroDeclSyntax.self)!.checkDefinition()
+    guard case let .expansion(_, replacements) = definition else {
+      XCTFail("not an expansion definition")
+      fatalError()
+    }
+
+    XCTAssertEqual(replacements.count, 2)
+    XCTAssertEqual(replacements[0].parameterIndex, 1)
+    XCTAssertEqual(replacements[1].parameterIndex, 0)
+  }
+
+  func testMacroDefinitionBad() throws {
+    let macro: DeclSyntax =
+      """
+      macro expand1(a: Int, b: Int) = #otherMacro(first: b + 1, c)
+      """
+
+    let diags: [Diagnostic]
+    do {
+      _ = try macro.as(MacroDeclSyntax.self)!.checkDefinition()
+      XCTFail("should have failed with an error")
+      fatalError()
+    } catch let diagError as DiagnosticsError {
+      diags = diagError.diagnostics
+    }
+
+    XCTAssertEqual(diags.count, 2)
+    XCTAssertEqual(
+      diags[0].diagMessage.message,
+      "only literals and macro parameters are permitted in expansion"
+    )
+    XCTAssertEqual(
+      diags[1].diagMessage.message,
+      "reference to value 'c' that is not a macro parameter in expansion"
+    )
+  }
+
+  func testMacroUndefined() throws {
+    let macro: DeclSyntax =
+      """
+      macro expand1(a: Int, b: Int)
+      """
+
+    let diags: [Diagnostic]
+    do {
+      _ = try macro.as(MacroDeclSyntax.self)!.checkDefinition()
+      XCTFail("should have failed with an error")
+      fatalError()
+    } catch let diagError as DiagnosticsError {
+      diags = diagError.diagnostics
+    }
+
+    XCTAssertEqual(diags.count, 1)
+    XCTAssertEqual(
+      diags[0].diagMessage.message,
+      "macro expansion requires a definition"
+    )
+  }
+
+  func testMacroExpansion() throws {
+    let macro: DeclSyntax =
+      """
+      macro expand1(a: Int, b: Int) = #otherMacro(first: b, second: ["a": a], third: [3.14159, 2.71828], fourth: 4)
+      """
+
+    let use: ExprSyntax =
+      """
+      #expand1(a: 5, b: 17)
+      """
+
+    let macroDecl = macro.as(MacroDeclSyntax.self)!
+    let definition = try macroDecl.checkDefinition()
+    guard case let .expansion(expansion, replacements) = definition else {
+      XCTFail("not a normal expansion")
+      fatalError()
+    }
+
+    let expandedSyntax = macroDecl.expand(
+      use.as(MacroExpansionExprSyntax.self)!,
+      definition: expansion,
+      replacements: replacements
+    )
+    AssertStringsEqualWithDiff(
+      expandedSyntax.description,
+      """
+      #otherMacro(first: 17, second: ["a": 5], third: [3.14159, 2.71828], fourth: 4)
+      """
+    )
+  }
+}