diff --git a/promise/polyfill.js b/promise/polyfill.js
index 4d990dcd..68cd60e1 100644
--- a/promise/polyfill.js
+++ b/promise/polyfill.js
@@ -61,6 +61,20 @@ PromisePolyfill.prototype.then = function(onFulfilled, onRejection) {
 PromisePolyfill.prototype.catch = function(onRejection) {
 	return this.then(null, onRejection)
 }
+PromisePolyfill.prototype.finally = function(callback) {
+	return this.then(
+		function(value) {
+			return PromisePolyfill.resolve(callback()).then(function() {
+				return value
+			})
+		},
+		function(reason) {
+			return PromisePolyfill.resolve(callback()).then(function() {
+				return PromisePolyfill.reject(reason);
+			})
+		}
+	)
+}
 PromisePolyfill.resolve = function(value) {
 	if (value instanceof PromisePolyfill) return value
 	return new PromisePolyfill(function(resolve) {resolve(value)})
diff --git a/promise/promise.js b/promise/promise.js
index 1f19bc33..7c6a2d11 100644
--- a/promise/promise.js
+++ b/promise/promise.js
@@ -3,10 +3,18 @@
 var PromisePolyfill = require("./polyfill")
 
 if (typeof window !== "undefined") {
-	if (typeof window.Promise === "undefined") window.Promise = PromisePolyfill
+	if (typeof window.Promise === "undefined") {
+		window.Promise = PromisePolyfill
+	} else if (!window.Promise.prototype.finally) {
+		window.Promise.prototype.finally = PromisePolyfill.prototype.finally
+	}
 	module.exports = window.Promise
 } else if (typeof global !== "undefined") {
-	if (typeof global.Promise === "undefined") global.Promise = PromisePolyfill
+	if (typeof global.Promise === "undefined") {
+		global.Promise = PromisePolyfill
+	} else if (!global.Promise.prototype.finally) {
+		global.Promise.prototype.finally = PromisePolyfill.prototype.finally
+	}
 	module.exports = global.Promise
 } else {
 	module.exports = PromisePolyfill
diff --git a/promise/tests/test-promise.js b/promise/tests/test-promise.js
index 2905d694..7dcc21f6 100644
--- a/promise/tests/test-promise.js
+++ b/promise/tests/test-promise.js
@@ -15,6 +15,7 @@ o.spec("promise", function() {
 		o("constructor has correct methods", function() {
 			o(typeof Promise.prototype.then).equals("function")
 			o(typeof Promise.prototype.catch).equals("function")
+			o(typeof Promise.prototype.finally).equals("function")
 			o(typeof Promise.resolve).equals("function")
 			o(typeof Promise.reject).equals("function")
 			o(typeof Promise.race).equals("function")
@@ -53,6 +54,78 @@ o.spec("promise", function() {
 				o(value).equals(1)
 			}).then(done)
 		})
+		o("finally lets a fulfilled value pass though", function(done) {
+			var promise = Promise.resolve(1)
+			var spy = o.spy(function(){return 2})
+
+			promise.finally(spy).then(function(value){
+				o(value).equals(1)
+				o(spy.callCount).equals(1)
+				o(spy.args.length).equals(0)
+				o(spy.this).equals(undefined)
+				done()
+			})
+		})
+		o("finally lets a rejected reason pass though", function(done) {
+			var promise = Promise.reject(1)
+			var spy = o.spy(function(){return 2})
+
+			promise.finally(spy).catch(function(reason){
+				o(reason).equals(1)
+				o(spy.callCount).equals(1)
+				o(spy.args.length).equals(0)
+				o(spy.this).equals(undefined)
+				done()
+			})
+		})
+		o("finally overrrides a fulfilled value when it throws", function(done) {
+			var promise = Promise.resolve(1)
+			var spy = o.spy(function(){throw 2})
+
+			promise.finally(spy).catch(function(reason){
+				o(reason).equals(2)
+				o(spy.callCount).equals(1)
+				o(spy.args.length).equals(0)
+				o(spy.this).equals(undefined)
+				done()
+			})
+		})
+		o("finally overrrides a fulfilled value when it returns a rejected Promise", function(done) {
+			var promise = Promise.resolve(1)
+			var spy = o.spy(function(){return Promise.reject(2)})
+
+			promise.finally(spy).catch(function(reason){
+				o(reason).equals(2)
+				o(spy.callCount).equals(1)
+				o(spy.args.length).equals(0)
+				o(spy.this).equals(undefined)
+				done()
+			})
+		})
+		o("finally overrrides a rejected reason when it throws", function(done) {
+			var promise = Promise.reject(1)
+			var spy = o.spy(function(){throw 2})
+
+			promise.finally(spy).catch(function(reason){
+				o(reason).equals(2)
+				o(spy.callCount).equals(1)
+				o(spy.args.length).equals(0)
+				o(spy.this).equals(undefined)
+				done()
+			})
+		})
+		o("finally overrrides a rejected reason when it returns a rejected Promise", function(done) {
+			var promise = Promise.reject(1)
+			var spy = o.spy(function(){return Promise.reject(2)})
+
+			promise.finally(spy).catch(function(reason){
+				o(reason).equals(2)
+				o(spy.callCount).equals(1)
+				o(spy.args.length).equals(0)
+				o(spy.this).equals(undefined)
+				done()
+			})
+		})
 	})
 	o.spec("resolve", function() {
 		o("resolves once", function(done) {