diff --git a/src/functions.php b/src/functions.php
index 350f914..0f2ef7e 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -3,6 +3,7 @@
 namespace React\Async;
 
 use React\EventLoop\Loop;
+use React\Promise\CancellablePromiseInterface;
 use React\Promise\Deferred;
 use React\Promise\PromiseInterface;
 
@@ -96,46 +97,53 @@ function ($error) use (&$exception, &$rejected, &$wait) {
  */
 function parallel(array $tasks)
 {
-    $deferred = new Deferred();
-    $results = array();
-    $errors = array();
-
-    $done = function () use (&$results, &$errors, $deferred) {
-        if (count($errors)) {
-            $deferred->reject(array_shift($errors));
-            return;
+    $pending = array();
+    $deferred = new Deferred(function () use (&$pending) {
+        foreach ($pending as $promise) {
+            if ($promise instanceof CancellablePromiseInterface) {
+                $promise->cancel();
+            }
         }
-
-        $deferred->resolve($results);
-    };
+        $pending = array();
+    });
+    $results = array();
+    $errored = false;
 
     $numTasks = count($tasks);
-
     if (0 === $numTasks) {
-        $done();
+        $deferred->resolve($results);
     }
 
-    $checkDone = function () use (&$results, &$errors, $numTasks, $done) {
-        if ($numTasks === count($results) + count($errors)) {
-            $done();
-        }
-    };
+    $taskErrback = function ($error) use (&$pending, $deferred, &$errored) {
+        $errored = true;
+        $deferred->reject($error);
 
-    $taskErrback = function ($error) use (&$errors, $checkDone) {
-        $errors[] = $error;
-        $checkDone();
+        foreach ($pending as $promise) {
+            if ($promise instanceof CancellablePromiseInterface) {
+                $promise->cancel();
+            }
+        }
+        $pending = array();
     };
 
     foreach ($tasks as $i => $task) {
-        $taskCallback = function ($result) use (&$results, $i, $checkDone) {
+        $taskCallback = function ($result) use (&$results, &$pending, $numTasks, $i, $deferred) {
             $results[$i] = $result;
-            $checkDone();
+
+            if (count($results) === $numTasks) {
+                $deferred->resolve($results);
+            }
         };
 
         $promise = call_user_func($task);
         assert($promise instanceof PromiseInterface);
+        $pending[$i] = $promise;
 
         $promise->then($taskCallback, $taskErrback);
+
+        if ($errored) {
+            break;
+        }
     }
 
     return $deferred->promise();
@@ -147,7 +155,13 @@ function parallel(array $tasks)
  */
 function series(array $tasks)
 {
-    $deferred = new Deferred();
+    $pending = null;
+    $deferred = new Deferred(function () use (&$pending) {
+        if ($pending instanceof CancellablePromiseInterface) {
+            $pending->cancel();
+        }
+        $pending = null;
+    });
     $results = array();
 
     /** @var callable():void $next */
@@ -156,7 +170,7 @@ function series(array $tasks)
         $next();
     };
 
-    $next = function () use (&$tasks, $taskCallback, $deferred, &$results) {
+    $next = function () use (&$tasks, $taskCallback, $deferred, &$results, &$pending) {
         if (0 === count($tasks)) {
             $deferred->resolve($results);
             return;
@@ -165,6 +179,7 @@ function series(array $tasks)
         $task = array_shift($tasks);
         $promise = call_user_func($task);
         assert($promise instanceof PromiseInterface);
+        $pending = $promise;
 
         $promise->then($taskCallback, array($deferred, 'reject'));
     };
@@ -180,10 +195,16 @@ function series(array $tasks)
  */
 function waterfall(array $tasks)
 {
-    $deferred = new Deferred();
+    $pending = null;
+    $deferred = new Deferred(function () use (&$pending) {
+        if ($pending instanceof CancellablePromiseInterface) {
+            $pending->cancel();
+        }
+        $pending = null;
+    });
 
     /** @var callable $next */
-    $next = function ($value = null) use (&$tasks, &$next, $deferred) {
+    $next = function ($value = null) use (&$tasks, &$next, $deferred, &$pending) {
         if (0 === count($tasks)) {
             $deferred->resolve($value);
             return;
@@ -192,6 +213,7 @@ function waterfall(array $tasks)
         $task = array_shift($tasks);
         $promise = call_user_func_array($task, func_get_args());
         assert($promise instanceof PromiseInterface);
+        $pending = $promise;
 
         $promise->then($next, array($deferred, 'reject'));
     };
diff --git a/tests/ParallelTest.php b/tests/ParallelTest.php
index f27a73e..b77a3ca 100644
--- a/tests/ParallelTest.php
+++ b/tests/ParallelTest.php
@@ -29,7 +29,7 @@ function () {
             },
             function () {
                 return new Promise(function ($resolve) {
-                    Loop::addTimer(0.1, function () use ($resolve) {
+                    Loop::addTimer(0.11, function () use ($resolve) {
                         $resolve('bar');
                     });
                 });
@@ -49,7 +49,7 @@ function () {
         $timer->assertInRange(0.1, 0.2);
     }
 
-    public function testParallelWithError()
+    public function testParallelWithErrorReturnsPromiseRejectedWithExceptionFromTaskAndStopsCallingAdditionalTasks()
     {
         $called = 0;
 
@@ -60,7 +60,8 @@ function () use (&$called) {
                     $resolve('foo');
                 });
             },
-            function () {
+            function () use (&$called) {
+                $called++;
                 return new Promise(function () {
                     throw new \RuntimeException('whoops');
                 });
@@ -80,7 +81,59 @@ function () use (&$called) {
         $this->assertSame(2, $called);
     }
 
-    public function testParallelWithDelayedError()
+    public function testParallelWithErrorWillCancelPendingPromises()
+    {
+        $cancelled = 0;
+
+        $tasks = array(
+            function () use (&$cancelled) {
+                return new Promise(function () { }, function () use (&$cancelled) {
+                    $cancelled++;
+                });
+            },
+            function () {
+                return new Promise(function () {
+                    throw new \RuntimeException('whoops');
+                });
+            },
+            function () use (&$cancelled) {
+                return new Promise(function () { }, function () use (&$cancelled) {
+                    $cancelled++;
+                });
+            }
+        );
+
+        $promise = React\Async\parallel($tasks);
+
+        $promise->then(null, $this->expectCallableOnceWith(new \RuntimeException('whoops')));
+
+        $this->assertSame(1, $cancelled);
+    }
+
+    public function testParallelWillCancelPendingPromisesWhenCallingCancelOnResultingPromise()
+    {
+        $cancelled = 0;
+
+        $tasks = array(
+            function () use (&$cancelled) {
+                return new Promise(function () { }, function () use (&$cancelled) {
+                    $cancelled++;
+                });
+            },
+            function () use (&$cancelled) {
+                return new Promise(function () { }, function () use (&$cancelled) {
+                    $cancelled++;
+                });
+            }
+        );
+
+        $promise = React\Async\parallel($tasks);
+        $promise->cancel();
+
+        $this->assertSame(2, $cancelled);
+    }
+
+    public function testParallelWithDelayedErrorReturnsPromiseRejectedWithExceptionFromTask()
     {
         $called = 0;
 
@@ -91,7 +144,8 @@ function () use (&$called) {
                     $resolve('foo');
                 });
             },
-            function () {
+            function () use (&$called) {
+                $called++;
                 return new Promise(function ($_, $reject) {
                     Loop::addTimer(0.001, function () use ($reject) {
                         $reject(new \RuntimeException('whoops'));
@@ -112,6 +166,6 @@ function () use (&$called) {
 
         Loop::run();
 
-        $this->assertSame(2, $called);
+        $this->assertSame(3, $called);
     }
 }
diff --git a/tests/SeriesTest.php b/tests/SeriesTest.php
index 2e168c2..71fc432 100644
--- a/tests/SeriesTest.php
+++ b/tests/SeriesTest.php
@@ -79,4 +79,27 @@ function () use (&$called) {
 
         $this->assertSame(1, $called);
     }
+
+    public function testSeriesWillCancelFirstPendingPromiseWhenCallingCancelOnResultingPromise()
+    {
+        $cancelled = 0;
+
+        $tasks = array(
+            function () {
+                return new Promise(function ($resolve) {
+                    $resolve();
+                });
+            },
+            function () use (&$cancelled) {
+                return new Promise(function () { }, function () use (&$cancelled) {
+                    $cancelled++;
+                });
+            }
+        );
+
+        $promise = React\Async\series($tasks);
+        $promise->cancel();
+
+        $this->assertSame(1, $cancelled);
+    }
 }
diff --git a/tests/WaterfallTest.php b/tests/WaterfallTest.php
index a04ad5c..b0c5c3c 100644
--- a/tests/WaterfallTest.php
+++ b/tests/WaterfallTest.php
@@ -86,4 +86,27 @@ function () use (&$called) {
 
         $this->assertSame(1, $called);
     }
+
+    public function testWaterfallWillCancelFirstPendingPromiseWhenCallingCancelOnResultingPromise()
+    {
+        $cancelled = 0;
+
+        $tasks = array(
+            function () {
+                return new Promise(function ($resolve) {
+                    $resolve();
+                });
+            },
+            function () use (&$cancelled) {
+                return new Promise(function () { }, function () use (&$cancelled) {
+                    $cancelled++;
+                });
+            }
+        );
+
+        $promise = React\Async\waterfall($tasks);
+        $promise->cancel();
+
+        $this->assertSame(1, $cancelled);
+    }
 }