diff --git a/README.md b/README.md
index a0fa118..ea86a0b 100644
--- a/README.md
+++ b/README.md
@@ -67,9 +67,9 @@ Optionally takes a callback to call when async tasks are complete.
 * `Promise` returned
   - Completion: [onFulfilled][promise-onfulfilled] method called
   - Error: [onRejected][promise-onrejected] method called
-* `Observable` returned
-  - Completion: [onCompleted][observable-subscribe] method called
-  - Error: [onError][observable-subscribe] method called
+* `Observable` (e.g. from [RxJS v5][rxjs5-observable] or [RxJS v4][rxjs5-observable]) returned
+  - Completion: [complete][rxjs5-subscriber-complete] method called
+  - Error: [error][rxjs5-subscriber-error] method called
 
 __Warning:__ Sync tasks are __not supported__ and your function will never complete if the one of the above strategies is not used to signal completion. However, thrown errors will be caught by the domain.
 
@@ -96,7 +96,10 @@ MIT
 [event-stream]: https://github.com/dominictarr/event-stream
 [promise-onfulfilled]: http://promisesaplus.com/#point-26
 [promise-onrejected]: http://promisesaplus.com/#point-30
-[observable-subscribe]: https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/operators/subscribe.md
+[rx4-observable]: https://github.com/Reactive-Extensions/RxJS/blob/master/doc/api/core/observable.md
+[rxjs5-observable]: http://reactivex.io/rxjs/class/es6/Observable.js~Observable.html
+[rxjs5-observer-complete]: http://reactivex.io/rxjs/class/es6/MiscJSDoc.js~ObserverDoc.html#instance-method-complete
+[rxjs5-observer-error]: http://reactivex.io/rxjs/class/es6/MiscJSDoc.js~ObserverDoc.html#instance-method-error
 
 [downloads-image]: http://img.shields.io/npm/dm/async-done.svg
 [npm-url]: https://www.npmjs.com/package/async-done
diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..2c4ab93
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,101 @@
+/**
+ * Notes about these type definitions:
+ *
+ * - Callbacks returning multiple completion values using multiple arguments are not supported by these types.
+ *   Prefer to use Node's style by grouping your values in a single object or array.
+ *   Support for this kind of callback is blocked by Microsoft/TypeScript#5453
+ *
+ * - For ease of use, `asyncDone` lets you pass callback functions with a result type `T` instead of `T | undefined`.
+ *   This matches Node's types but can lead to unsound code being typechecked.
+ *
+ *   The following code typechecks but fails at runtime:
+ *   ```typescript
+ *   async function getString(): Promise<string> {
+ *     return "Hello, World!";
+ *   }
+ *
+ *   async function evilGetString(): Promise<string> {
+ *     throw new Error("Hello, World!");
+ *   }
+ *
+ *   function cb(err: Error | null, result: string): void {
+ *     // This is unsound because `result` is `undefined` when `err` is not `null`.
+ *     console.log(result.toLowerCase());
+ *   }
+ *
+ *   asyncDone(getString, cb); // Prints `hello, world!`
+ *   asyncDone(evilGetString, cb); // Runtime error: `TypeError: Cannot read property 'toLowerCase' of undefined`
+ *   ```
+ *
+ *   Enforcing stricter callbacks would require developers to use `result?: string` and assert the existence
+ *   of the result either by checking it directly or using the `!` assertion operator after testing for errors.
+ *   ```typescript
+ *   function stricterCb1(err: Error | null, result?: string): void {
+ *     if (err !== null) {
+ *       console.error(err);
+ *       return;
+ *     }
+ *     console.log(result!.toLowerCase());
+ *   }
+ *
+ *   function stricterCb2(err: Error | null, result?: string): void {
+ *     if (result === undefined) {
+ *       console.error("Undefined result. Error:);
+ *       console.error(err);
+ *       return;
+ *     }
+ *     console.log(result.toLowerCase());
+ *   }
+ *   ```
+ */
+import { ChildProcess } from "child_process";
+import { EventEmitter } from "events";
+import { Stream } from "stream";
+
+declare namespace asyncDone {
+
+  /**
+   * Represents a callback function used to signal the completion of a
+   * task without any result value.
+   */
+  type VoidCallback = (err: Error | null) => void;
+
+  /**
+   * Represents a callback function used to signal the completion of a
+   * task with a single result value.
+   */
+  interface Callback<T> {
+    (err: null, result: T): void;
+
+    // Use `result?: T` or `result: undefined` to require the consumer to assert the existence of the result
+    // (even in case of success). See comment at the top of the file.
+    (err: Error, result?: any): void;
+  }
+
+  /**
+   * Minimal `Observable` interface compatible with `async-done`.
+   *
+   * @see https://github.com/ReactiveX/rxjs/blob/c3c56867eaf93f302ac7cd588034c7d8712f2834/src/internal/Observable.ts#L77
+   */
+  interface Observable<T = any> {
+    subscribe(next?: (value: T) => void, error?: (error: any) => void, complete?: () => void): any;
+  }
+
+  /**
+   * Represents an async operation.
+   */
+  export type AsyncTask<R = any> =
+    ((done: VoidCallback) => void)
+    | ((done: Callback<R>) => void)
+    | (() => ChildProcess | EventEmitter | Observable<R> | PromiseLike<R> | Stream);
+}
+
+/**
+ * Takes a function to execute (`fn`) and a function to call on completion (`callback`).
+ *
+ * @param fn Function to execute.
+ * @param callback Function to call on completion.
+ */
+declare function asyncDone<R = any>(fn: asyncDone.AsyncTask<R>, callback: asyncDone.Callback<R>): void;
+
+export = asyncDone;
diff --git a/package.json b/package.json
index eebaad2..6e10e16 100644
--- a/package.json
+++ b/package.json
@@ -6,7 +6,8 @@
   "contributors": [
     "Blaine Bublitz <blaine.bublitz@gmail.com>",
     "Pawel Kozlowski <pkozlowski.opensource@gmail.com>",
-    "Matthew Podwysocki <matthew.podwysocki@gmail.com>"
+    "Matthew Podwysocki <matthew.podwysocki@gmail.com>",
+    "Charles Samborski <demurgos@demurgos.net>"
   ],
   "repository": "gulpjs/async-done",
   "license": "MIT",
@@ -14,6 +15,7 @@
     "node": ">= 0.10"
   },
   "main": "index.js",
+  "types": "index.d.ts",
   "files": [
     "index.js",
     "LICENSE"
@@ -21,7 +23,8 @@
   "scripts": {
     "lint": "eslint . && jscs index.js test/",
     "pretest": "npm run lint",
-    "test": "mocha --async-only",
+    "test": "mocha --async-only && npm run test-types",
+    "test-types": "tsc -p test/types",
     "cover": "istanbul cover _mocha --report lcovonly",
     "coveralls": "npm run cover && istanbul-coveralls"
   },
@@ -32,6 +35,7 @@
     "stream-exhaust": "^1.0.1"
   },
   "devDependencies": {
+    "@types/node": "^9.3.0",
     "eslint": "^1.7.3",
     "eslint-config-gulp": "^2.0.0",
     "expect": "^1.19.0",
@@ -41,8 +45,9 @@
     "jscs-preset-gulp": "^1.0.0",
     "mocha": "^2.4.5",
     "pumpify": "^1.3.6",
-    "rx": "^4.0.6",
+    "rxjs": "^5.5.6",
     "through2": "^2.0.0",
+    "typescript": "^2.6.2",
     "when": "^3.7.3"
   },
   "keywords": [
diff --git a/test/observables.js b/test/observables.js
index 797b994..4956878 100644
--- a/test/observables.js
+++ b/test/observables.js
@@ -4,14 +4,15 @@ var expect = require('expect');
 
 var asyncDone = require('../');
 
-var Observable = require('rx').Observable;
+var Observable = require('rxjs').Observable;
 
 function success() {
   return Observable.empty();
 }
 
 function successValue() {
-  return Observable.return(42);
+  // This corresponds to `Observable.return(42);` in RxJS 4
+  return Observable.of(42);
 }
 
 function failure() {
diff --git a/test/types/callback.ts b/test/types/callback.ts
new file mode 100644
index 0000000..17a0651
--- /dev/null
+++ b/test/types/callback.ts
@@ -0,0 +1,44 @@
+import asyncDone, { Callback } from "async-done";
+
+function success(cb: Callback<number>): void {
+  cb(null, 2);
+}
+
+function failure(cb: Callback<number>): void {
+  cb(new Error("Callback Error"));
+}
+
+function neverDone(): number {
+  return 2;
+}
+
+// `success` and stricter callback
+asyncDone(success, function (err: Error | null, result?: number): void {
+  console.log("Done");
+});
+
+// The following code fails to compile as expected:
+// asyncDone(success, function (err: Error | null, result?: string): void {
+//   console.log("Done");
+// });
+
+// `success` and unsound callback
+asyncDone(success, function (err: Error | null, result: number): void {
+  console.log("Done");
+});
+
+// `failure` and stricter callback
+asyncDone(failure, function (err: Error | null, result?: number): void {
+  console.log("Done");
+});
+
+// `failure` and unsound callback
+asyncDone(failure, function (err: Error | null, result: number): void {
+  console.log("Done");
+});
+
+// I don't think TS is currently able to prevent the current code from compiling
+// (`neverDone` matches with `(done: VoidCallback) => void` for example)
+// asyncDone(neverDone, function(err: Error | null, result?: number): void {
+//  console.log("Done");
+// });
diff --git a/test/types/child_processes.ts b/test/types/child_processes.ts
new file mode 100644
index 0000000..d0d431b
--- /dev/null
+++ b/test/types/child_processes.ts
@@ -0,0 +1,19 @@
+import asyncDone from "async-done";
+import cp from "child_process";
+
+
+function success(): cp.ChildProcess {
+  return cp.exec("echo hello world");
+}
+
+function failure(): cp.ChildProcess {
+  return cp.exec("foo-bar-baz hello world");
+}
+
+asyncDone(success, function (err: Error | null): void {
+  console.log("Done");
+});
+
+asyncDone(failure, function (err: Error | null): void {
+  console.log("Done");
+});
diff --git a/test/types/observables.ts b/test/types/observables.ts
new file mode 100644
index 0000000..4f0dd53
--- /dev/null
+++ b/test/types/observables.ts
@@ -0,0 +1,46 @@
+import asyncDone from "async-done";
+import { Observable } from "rxjs/Observable";
+import 'rxjs/add/observable/empty';
+import 'rxjs/add/observable/of';
+
+function success(): Observable<undefined> {
+  return Observable.empty();
+}
+
+function successValue(): Observable<number> {
+  return Observable.of(42);
+}
+
+function failure(): Observable<number> {
+  return Observable.throw(new Error("Observable error"));
+}
+
+// `success` callback
+asyncDone(success, function (err: Error | null): void {
+  console.log("Done");
+});
+
+// The following code fails to compile as expected (`undefined` is not assignable to `number`):
+// asyncDone(success, function (err: Error | null, result: number): void {
+//   console.log("Done");
+// });
+
+// `successValue` and stricter callback
+asyncDone(successValue, function (err: Error | null, result?: number): void {
+  console.log("Done");
+});
+
+// `successValue` and unsound callback
+asyncDone(successValue, function (err: Error | null, result: number): void {
+  console.log("Done");
+});
+
+// `failure` and stricter callback
+asyncDone(failure, function (err: Error | null, result?: number): void {
+  console.log("Done");
+});
+
+// `failure` and unsound callback
+asyncDone(failure, function (err: Error | null, result: number): void {
+  console.log("Done");
+});
diff --git a/test/types/promises.ts b/test/types/promises.ts
new file mode 100644
index 0000000..e7a7498
--- /dev/null
+++ b/test/types/promises.ts
@@ -0,0 +1,34 @@
+import asyncDone from "async-done";
+
+function success(): Promise<number> {
+  return Promise.resolve(2);
+}
+
+function failure(): Promise<number> {
+  return Promise.reject(new Error("Promise Error"));
+}
+
+// `successValue` and stricter callback
+asyncDone(success, function (err: Error | null, result?: number): void {
+  console.log("Done");
+});
+
+// The following code fails to compile as expected:
+// asyncDone(success, function (err: Error | null, result?: string): void {
+//   console.log("Done");
+// });
+
+// `successValue` and unsound callback
+asyncDone(success, function (err: Error | null, result: number): void {
+  console.log("Done");
+});
+
+// `failure` and stricter callback
+asyncDone(failure, function (err: Error | null, result?: number): void {
+  console.log("Done");
+});
+
+// `failure` and unsound callback
+asyncDone(failure, function (err: Error | null, result: number): void {
+  console.log("Done");
+});
diff --git a/test/types/streams.ts b/test/types/streams.ts
new file mode 100644
index 0000000..cda238b
--- /dev/null
+++ b/test/types/streams.ts
@@ -0,0 +1,18 @@
+import asyncDone from "async-done";
+import { Readable, Stream } from "stream";
+
+function streamSuccess(): Stream {
+  return new Stream();
+}
+
+function streamFail(): Stream {
+  return new Stream();
+}
+
+asyncDone(streamSuccess, function (err: Error | null): void {
+  console.log("Done");
+});
+
+asyncDone(streamFail, function (err: Error | null): void {
+  console.log("Done");
+});
diff --git a/test/types/tsconfig.json b/test/types/tsconfig.json
new file mode 100644
index 0000000..35ef644
--- /dev/null
+++ b/test/types/tsconfig.json
@@ -0,0 +1,63 @@
+{
+  "compilerOptions": {
+    "allowJs": false,
+    "allowSyntheticDefaultImports": true,
+    "allowUnreachableCode": false,
+    "allowUnusedLabels": false,
+    "alwaysStrict": true,
+    "baseUrl": "../..",
+    "charset": "utf8",
+    "checkJs": false,
+    "declaration": true,
+    "disableSizeLimit": false,
+    "downlevelIteration": false,
+    "emitBOM": false,
+    "emitDecoratorMetadata": true,
+    "experimentalDecorators": true,
+    "forceConsistentCasingInFileNames": true,
+    "importHelpers": false,
+    "inlineSourceMap": false,
+    "inlineSources": false,
+    "isolatedModules": false,
+    "lib": [
+      "es2017"
+    ],
+    "locale": "en-us",
+    "module": "commonjs",
+    "moduleResolution": "node",
+    "newLine": "lf",
+    "noEmit": true,
+    "noEmitHelpers": false,
+    "noEmitOnError": true,
+    "noErrorTruncation": true,
+    "noFallthroughCasesInSwitch": true,
+    "noImplicitAny": true,
+    "noImplicitReturns": true,
+    "noImplicitThis": true,
+    "noStrictGenericChecks": false,
+    "noUnusedLocals": false,
+    "noUnusedParameters": false,
+    "noLib": false,
+    "noResolve": false,
+    "paths": {
+      "async-done": [
+        "index.d.ts"
+      ]
+    },
+    "preserveConstEnums": true,
+    "removeComments": false,
+    "rootDir": "",
+    "skipLibCheck": false,
+    "sourceMap": true,
+    "strict": true,
+    "strictNullChecks": true,
+    "suppressExcessPropertyErrors": false,
+    "suppressImplicitAnyIndexErrors": false,
+    "target": "es2017",
+    "traceResolution": false
+  },
+  "include": [
+    "./**/*.ts"
+  ],
+  "exclude": []
+}
diff --git a/test/types/various.ts b/test/types/various.ts
new file mode 100644
index 0000000..73c6956
--- /dev/null
+++ b/test/types/various.ts
@@ -0,0 +1,5 @@
+import asyncDone, {AsyncTask, VoidCallback} from "async-done";
+
+// Do not error if the return value is not `void`.
+const fn: AsyncTask = (cb: VoidCallback): NodeJS.Timer => setTimeout(cb, 1000);
+asyncDone(fn, () => {});