diff --git a/lib/index.js b/lib/index.js
index cdd7102..73ef3f3 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -71,7 +71,7 @@ function configure(settings) {
     extension = extensions[index]
     unsafe = unsafe.concat(extension.unsafe || [])
     join = join.concat(extension.join || [])
-    handlers = Object.assign(handlers, extension.handlers || {})
+    Object.assign(handlers, extension.handlers || {})
   }
 
   return {unsafe: unsafe, join: join, handlers: handlers}
diff --git a/package.json b/package.json
index 5b3b1ae..483acc4 100644
--- a/package.json
+++ b/package.json
@@ -29,10 +29,13 @@
     "Titus Wormer <tituswormer@gmail.com> (https://wooorm.com)"
   ],
   "files": [
+    "index.js",
     "lib/",
-    "index.js"
+    "types/index.d.ts"
   ],
+  "types": "types",
   "dependencies": {
+    "@types/unist": "^2.0.0",
     "longest-streak": "^2.0.0",
     "mdast-util-to-string": "^1.0.0",
     "parse-entities": "^2.0.0",
@@ -42,6 +45,7 @@
   },
   "devDependencies": {
     "browserify": "^17.0.0",
+    "dtslint": "^4.0.0",
     "mdast-util-from-markdown": "^0.8.0",
     "nyc": "^15.0.0",
     "prettier": "^2.0.0",
@@ -56,7 +60,8 @@
     "build": "browserify . -s mdastUtilToMarkdown -p tinyify > mdast-util-to-markdown.min.js",
     "test-api": "node test",
     "test-coverage": "nyc --reporter lcov tape test.js",
-    "test": "npm run format && npm run build && npm run test-coverage"
+    "test-types": "dtslint types",
+    "test": "npm run format && npm run build && npm run test-coverage && npm run test-types"
   },
   "nyc": {
     "check-coverage": true,
@@ -77,7 +82,10 @@
     "esnext": false,
     "rules": {
       "unicorn/prefer-includes": "off"
-    }
+    },
+    "ignores": [
+      "types/"
+    ]
   },
   "remarkConfig": {
     "plugins": [
diff --git a/types/index.d.ts b/types/index.d.ts
new file mode 100644
index 0000000..95e4e04
--- /dev/null
+++ b/types/index.d.ts
@@ -0,0 +1,87 @@
+// Minimum TypeScript Version: 3.0
+import {Node, Parent} from 'unist'
+
+export = toMarkdown
+
+declare namespace toMarkdown {
+  interface SafeOptions {
+    before: string
+    after: string
+  }
+
+  type Handle = (
+    node: Node,
+    parent: Parent | null | undefined,
+    context: Context,
+    safeOptions: SafeOptions
+  ) => string
+
+  interface Context {
+    stack: string[]
+    enter: (type: string) => () => void
+    options: Options
+    unsafePatterns: Unsafe[]
+    join: Join[]
+    handle: Handle
+  }
+
+  interface Handlers {
+    [key: string]: Handler
+  }
+
+  interface Handler {
+    peek?: Handle
+    (
+      node: Node,
+      parent: Parent | null | undefined,
+      context: Context,
+      safeOptions: SafeOptions
+    ): string
+  }
+
+  interface Unsafe {
+    character: string
+    inConstruct?: string | string[]
+    notInConstruct?: string | string[]
+    after?: string
+    before?: string
+    atBreak?: boolean
+  }
+
+  type Join = (
+    left: Node,
+    right: Node,
+    parent: Parent,
+    context: Context
+  ) => boolean | null | void
+
+  interface Extension {
+    handlers?: Handlers
+    join?: Join[]
+    unsafe?: Unsafe[]
+  }
+
+  interface Options {
+    bullet?: '-' | '*' | '+'
+    closeAtx?: boolean
+    emphasis?: '_' | '*'
+    fence?: '~' | '`'
+    fences?: boolean
+    incrementListMarker?: boolean
+    listItemIndent?: 'tab' | 'one' | 'mixed'
+    quote?: '"' | "'"
+    rule?: '-' | '_' | '*'
+    ruleRepeat?: number
+    ruleSpaces?: boolean
+    setext?: boolean
+    strong?: '_' | '*'
+    tightDefinitions?: boolean
+
+    extensions?: Extension[]
+    handlers?: Handlers
+    join?: Join[]
+    unsafe?: Unsafe[]
+  }
+}
+
+declare function toMarkdown(node: Node, options?: toMarkdown.Options): string
diff --git a/types/test.ts b/types/test.ts
new file mode 100644
index 0000000..b776991
--- /dev/null
+++ b/types/test.ts
@@ -0,0 +1,49 @@
+// This file is for https://github.com/microsoft/dtslint .
+// Tests are type-checked, but not run.
+
+import * as toMarkdown from 'mdast-util-to-markdown'
+
+function main() {
+  const node = {type: 'root'}
+  const handleOk = () => '\\\n'
+  const handleNok = () => 1
+  const joinOk = () => {}
+  const joinNok = () => {
+    return 1
+  }
+
+  joinOk.peek = () => '\\'
+
+  // $ExpectType string
+  toMarkdown(node)
+
+  // $ExpectError
+  toMarkdown(node, {unknown: '1'})
+
+  // $ExpectType string
+  toMarkdown(node, {bullet: '+'})
+  // $ExpectError
+  toMarkdown(node, {bullet: '?'})
+
+  // $ExpectType string
+  toMarkdown(node, {
+    unsafe: [
+      {atBreak: true, character: '_'},
+      {atBreak: true, before: '\\d+', character: '.', after: '(?:[ \t\r\n]|$)'}
+    ]
+  })
+  // $ExpectError
+  toMarkdown(node, {unsafe: [{unknown: true}]})
+
+  // $ExpectType string
+  toMarkdown(node, {join: [joinOk]})
+  // $ExpectError
+  toMarkdown(node, {join: [joinNok]})
+
+  // $ExpectType string
+  toMarkdown(node, {handlers: {break: handleOk}})
+  // $ExpectError
+  toMarkdown(node, {handlers: {break: handleNok}})
+}
+
+main()
diff --git a/types/tsconfig.json b/types/tsconfig.json
new file mode 100644
index 0000000..5677cb7
--- /dev/null
+++ b/types/tsconfig.json
@@ -0,0 +1,15 @@
+{
+  "compilerOptions": {
+    "moduleResolution": "node",
+    "lib": [
+      "ES5"
+    ],
+    "strict": true,
+    "baseUrl": ".",
+    "paths": {
+      "mdast-util-to-markdown": [
+        "./index.d.ts"
+      ]
+    }
+  }
+}
diff --git a/types/tslint.json b/types/tslint.json
new file mode 100644
index 0000000..70c4494
--- /dev/null
+++ b/types/tslint.json
@@ -0,0 +1,7 @@
+{
+  "extends": "dtslint/dtslint.json",
+  "rules": {
+    "semicolon": false,
+    "whitespace": false
+  }
+}