diff --git a/.travis.yml b/.travis.yml
index e176d1fc..bd2ae443 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,7 +1,6 @@
 language: php
 
 php:
-  - 7.0
   - 7.1
   - 7.2
   - 7.3
diff --git a/composer.json b/composer.json
index 02f958b6..f0b279d2 100644
--- a/composer.json
+++ b/composer.json
@@ -6,7 +6,7 @@
         {"name": "Jan Sorgalla", "email": "jsorgalla@gmail.com"}
     ],
     "require": {
-        "php": ">=7.0.0"
+        "php": ">=7.1.0"
     },
     "require-dev": {
         "phpunit/phpunit": "~6.4"
diff --git a/src/Deferred.php b/src/Deferred.php
index ef06069f..c748633f 100644
--- a/src/Deferred.php
+++ b/src/Deferred.php
@@ -14,13 +14,13 @@ public function __construct(callable $canceller = null)
         $this->canceller = $canceller;
     }
 
-    public function promise()
+    public function promise(): PromiseInterface
     {
         if (null === $this->promise) {
             $canceller = $this->canceller;
             $this->canceller = null;
 
-            $this->promise = new Promise(function ($resolve, $reject) {
+            $this->promise = new Promise(function ($resolve, $reject): void {
                 $this->resolveCallback = $resolve;
                 $this->rejectCallback  = $reject;
             }, $canceller);
@@ -29,14 +29,14 @@ public function promise()
         return $this->promise;
     }
 
-    public function resolve($value = null)
+    public function resolve($value = null): void
     {
         $this->promise();
 
         \call_user_func($this->resolveCallback, $value);
     }
 
-    public function reject(\Throwable $reason)
+    public function reject(\Throwable $reason): void
     {
         $this->promise();
 
diff --git a/src/Exception/CompositeException.php b/src/Exception/CompositeException.php
index 1fdbab54..022bd0ae 100644
--- a/src/Exception/CompositeException.php
+++ b/src/Exception/CompositeException.php
@@ -23,7 +23,7 @@ public function __construct(array $throwables, $message = '', $code = 0, $previo
     /**
      * @return \Throwable[]
      */
-    public function getThrowables()
+    public function getThrowables(): array
     {
         return $this->throwables;
     }
diff --git a/src/FulfilledPromise.php b/src/FulfilledPromise.php
index ecd09ba7..2bb0a060 100644
--- a/src/FulfilledPromise.php
+++ b/src/FulfilledPromise.php
@@ -15,14 +15,14 @@ public function __construct($value = null)
         $this->value = $value;
     }
 
-    public function then(callable $onFulfilled = null, callable $onRejected = null)
+    public function then(callable $onFulfilled = null, callable $onRejected = null): PromiseInterface
     {
         if (null === $onFulfilled) {
             return $this;
         }
 
-        return new Promise(function (callable $resolve, callable $reject) use ($onFulfilled) {
-            enqueue(function () use ($resolve, $reject, $onFulfilled) {
+        return new Promise(function (callable $resolve, callable $reject) use ($onFulfilled): void {
+            enqueue(function () use ($resolve, $reject, $onFulfilled): void {
                 try {
                     $resolve($onFulfilled($this->value));
                 } catch (\Throwable $exception) {
@@ -32,7 +32,7 @@ public function then(callable $onFulfilled = null, callable $onRejected = null)
         });
     }
 
-    public function done(callable $onFulfilled = null, callable $onRejected = null)
+    public function done(callable $onFulfilled = null, callable $onRejected = null): void
     {
         if (null === $onFulfilled) {
             return;
@@ -51,21 +51,21 @@ public function done(callable $onFulfilled = null, callable $onRejected = null)
         });
     }
 
-    public function otherwise(callable $onRejected)
+    public function otherwise(callable $onRejected): PromiseInterface
     {
         return $this;
     }
 
-    public function always(callable $onFulfilledOrRejected)
+    public function always(callable $onFulfilledOrRejected): PromiseInterface
     {
-        return $this->then(function ($value) use ($onFulfilledOrRejected) {
+        return $this->then(function ($value) use ($onFulfilledOrRejected): PromiseInterface {
             return resolve($onFulfilledOrRejected())->then(function () use ($value) {
                 return $value;
             });
         });
     }
 
-    public function cancel()
+    public function cancel(): void
     {
     }
 }
diff --git a/src/Internal/CancellationQueue.php b/src/Internal/CancellationQueue.php
index af22c68e..52abf6e2 100644
--- a/src/Internal/CancellationQueue.php
+++ b/src/Internal/CancellationQueue.php
@@ -10,7 +10,7 @@ final class CancellationQueue
     private $started = false;
     private $queue = [];
 
-    public function __invoke()
+    public function __invoke(): void
     {
         if ($this->started) {
             return;
@@ -20,7 +20,7 @@ public function __invoke()
         $this->drain();
     }
 
-    public function enqueue($cancellable)
+    public function enqueue($cancellable): void
     {
         if (!\method_exists($cancellable, 'then') || !\method_exists($cancellable, 'cancel')) {
             return;
@@ -33,7 +33,7 @@ public function enqueue($cancellable)
         }
     }
 
-    private function drain()
+    private function drain(): void
     {
         for ($i = \key($this->queue); isset($this->queue[$i]); $i++) {
             $cancellable = $this->queue[$i];
diff --git a/src/Internal/Queue.php b/src/Internal/Queue.php
index 6b4bfe0b..19939881 100644
--- a/src/Internal/Queue.php
+++ b/src/Internal/Queue.php
@@ -9,14 +9,14 @@ final class Queue
 {
     private $queue = [];
 
-    public function enqueue(callable $task)
+    public function enqueue(callable $task): void
     {
         if (1 === \array_push($this->queue, $task)) {
             $this->drain();
         }
     }
 
-    private function drain()
+    private function drain(): void
     {
         for ($i = \key($this->queue); isset($this->queue[$i]); $i++) {
             $task = $this->queue[$i];
diff --git a/src/Promise.php b/src/Promise.php
index f25d3667..824a46cc 100644
--- a/src/Promise.php
+++ b/src/Promise.php
@@ -17,7 +17,7 @@ public function __construct(callable $resolver, callable $canceller = null)
         $this->call($resolver);
     }
 
-    public function then(callable $onFulfilled = null, callable $onRejected = null)
+    public function then(callable $onFulfilled = null, callable $onRejected = null): PromiseInterface
     {
         if (null !== $this->result) {
             return $this->result->then($onFulfilled, $onRejected);
@@ -38,10 +38,11 @@ public function then(callable $onFulfilled = null, callable $onRejected = null)
         });
     }
 
-    public function done(callable $onFulfilled = null, callable $onRejected = null)
+    public function done(callable $onFulfilled = null, callable $onRejected = null): void
     {
         if (null !== $this->result) {
-            return $this->result->done($onFulfilled, $onRejected);
+            $this->result->done($onFulfilled, $onRejected);
+            return;
         }
 
         $this->handlers[] = function (PromiseInterface $promise) use ($onFulfilled, $onRejected) {
@@ -50,7 +51,7 @@ public function done(callable $onFulfilled = null, callable $onRejected = null)
         };
     }
 
-    public function otherwise(callable $onRejected)
+    public function otherwise(callable $onRejected): PromiseInterface
     {
         return $this->then(null, function ($reason) use ($onRejected) {
             if (!_checkTypehint($onRejected, $reason)) {
@@ -61,7 +62,7 @@ public function otherwise(callable $onRejected)
         });
     }
 
-    public function always(callable $onFulfilledOrRejected)
+    public function always(callable $onFulfilledOrRejected): PromiseInterface
     {
         return $this->then(function ($value) use ($onFulfilledOrRejected) {
             return resolve($onFulfilledOrRejected())->then(function () use ($value) {
@@ -74,7 +75,7 @@ public function always(callable $onFulfilledOrRejected)
         });
     }
 
-    public function cancel()
+    public function cancel(): void
     {
         $canceller = $this->canceller;
         $this->canceller = null;
@@ -109,7 +110,7 @@ public function cancel()
         }
     }
 
-    private function resolver(callable $onFulfilled = null, callable $onRejected = null)
+    private function resolver(callable $onFulfilled = null, callable $onRejected = null): callable
     {
         return function ($resolve, $reject) use ($onFulfilled, $onRejected) {
             $this->handlers[] = function (PromiseInterface $promise) use ($onFulfilled, $onRejected, $resolve, $reject) {
@@ -120,7 +121,7 @@ private function resolver(callable $onFulfilled = null, callable $onRejected = n
         };
     }
 
-    private function resolve($value = null)
+    private function resolve($value = null): void
     {
         if (null !== $this->result) {
             return;
@@ -129,7 +130,7 @@ private function resolve($value = null)
         $this->settle(resolve($value));
     }
 
-    private function reject(\Throwable $reason)
+    private function reject(\Throwable $reason): void
     {
         if (null !== $this->result) {
             return;
@@ -138,7 +139,7 @@ private function reject(\Throwable $reason)
         $this->settle(reject($reason));
     }
 
-    private function settle(PromiseInterface $result)
+    private function settle(PromiseInterface $result): void
     {
         $result = $this->unwrap($result);
 
@@ -165,7 +166,7 @@ private function settle(PromiseInterface $result)
         }
     }
 
-    private function unwrap($promise)
+    private function unwrap($promise): PromiseInterface
     {
         while ($promise instanceof self && null !== $promise->result) {
             $promise = $promise->result;
@@ -174,7 +175,7 @@ private function unwrap($promise)
         return $promise;
     }
 
-    private function call(callable $callback)
+    private function call(callable $callback): void
     {
         // Use reflection to inspect number of arguments expected by this callback.
         // We did some careful benchmarking here: Using reflection to avoid unneeded
diff --git a/src/PromiseInterface.php b/src/PromiseInterface.php
index 6612834a..3f854b94 100644
--- a/src/PromiseInterface.php
+++ b/src/PromiseInterface.php
@@ -32,7 +32,7 @@ interface PromiseInterface
      * @param callable|null $onRejected
      * @return PromiseInterface
      */
-    public function then(callable $onFulfilled = null, callable $onRejected = null);
+    public function then(?callable $onFulfilled = null, ?callable $onRejected = null): PromiseInterface;
 
     /**
      * Consumes the promise's ultimate value if the promise fulfills, or handles the
@@ -48,7 +48,7 @@ public function then(callable $onFulfilled = null, callable $onRejected = null);
      * @param callable|null $onRejected
      * @return void
      */
-    public function done(callable $onFulfilled = null, callable $onRejected = null);
+    public function done(callable $onFulfilled = null, callable $onRejected = null): void;
 
     /**
      * Registers a rejection handler for promise. It is a shortcut for:
@@ -63,7 +63,7 @@ public function done(callable $onFulfilled = null, callable $onRejected = null);
      * @param callable $onRejected
      * @return PromiseInterface
      */
-    public function otherwise(callable $onRejected);
+    public function otherwise(callable $onRejected): PromiseInterface;
 
     /**
      * Allows you to execute "cleanup" type tasks in a promise chain.
@@ -110,7 +110,7 @@ public function otherwise(callable $onRejected);
      * @param callable $onFulfilledOrRejected
      * @return PromiseInterface
      */
-    public function always(callable $onFulfilledOrRejected);
+    public function always(callable $onFulfilledOrRejected): PromiseInterface;
 
     /**
      * The `cancel()` method notifies the creator of the promise that there is no
@@ -121,5 +121,5 @@ public function always(callable $onFulfilledOrRejected);
      *
      * @return void
      */
-    public function cancel();
+    public function cancel(): void;
 }
diff --git a/src/PromisorInterface.php b/src/PromisorInterface.php
index bd644008..87e999c1 100644
--- a/src/PromisorInterface.php
+++ b/src/PromisorInterface.php
@@ -9,5 +9,5 @@ interface PromisorInterface
      *
      * @return PromiseInterface
      */
-    public function promise();
+    public function promise(): PromiseInterface;
 }
diff --git a/src/RejectedPromise.php b/src/RejectedPromise.php
index d256b98f..2da6d532 100644
--- a/src/RejectedPromise.php
+++ b/src/RejectedPromise.php
@@ -11,14 +11,14 @@ public function __construct(\Throwable $reason)
         $this->reason = $reason;
     }
 
-    public function then(callable $onFulfilled = null, callable $onRejected = null)
+    public function then(callable $onFulfilled = null, callable $onRejected = null): PromiseInterface
     {
         if (null === $onRejected) {
             return $this;
         }
 
-        return new Promise(function (callable $resolve, callable $reject) use ($onRejected) {
-            enqueue(function () use ($resolve, $reject, $onRejected) {
+        return new Promise(function (callable $resolve, callable $reject) use ($onRejected): void {
+            enqueue(function () use ($resolve, $reject, $onRejected): void {
                 try {
                     $resolve($onRejected($this->reason));
                 } catch (\Throwable $exception) {
@@ -28,7 +28,7 @@ public function then(callable $onFulfilled = null, callable $onRejected = null)
         });
     }
 
-    public function done(callable $onFulfilled = null, callable $onRejected = null)
+    public function done(callable $onFulfilled = null, callable $onRejected = null): void
     {
         enqueue(function () use ($onRejected) {
             if (null === $onRejected) {
@@ -51,7 +51,7 @@ public function done(callable $onFulfilled = null, callable $onRejected = null)
         });
     }
 
-    public function otherwise(callable $onRejected)
+    public function otherwise(callable $onRejected): PromiseInterface
     {
         if (!_checkTypehint($onRejected, $this->reason)) {
             return $this;
@@ -60,16 +60,16 @@ public function otherwise(callable $onRejected)
         return $this->then(null, $onRejected);
     }
 
-    public function always(callable $onFulfilledOrRejected)
+    public function always(callable $onFulfilledOrRejected): PromiseInterface
     {
-        return $this->then(null, function (\Throwable $reason) use ($onFulfilledOrRejected) {
-            return resolve($onFulfilledOrRejected())->then(function () use ($reason) {
+        return $this->then(null, function (\Throwable $reason) use ($onFulfilledOrRejected): PromiseInterface {
+            return resolve($onFulfilledOrRejected())->then(function () use ($reason): PromiseInterface {
                 return new RejectedPromise($reason);
             });
         });
     }
 
-    public function cancel()
+    public function cancel(): void
     {
     }
 }
diff --git a/src/functions.php b/src/functions.php
index 4f2dc54f..2439c77c 100644
--- a/src/functions.php
+++ b/src/functions.php
@@ -19,7 +19,7 @@
  * @return PromiseInterface
  */
 
-function resolve($promiseOrValue = null)
+function resolve($promiseOrValue = null): PromiseInterface
 {
     if ($promiseOrValue instanceof PromiseInterface) {
         return $promiseOrValue;
@@ -32,7 +32,7 @@ function resolve($promiseOrValue = null)
             $canceller = [$promiseOrValue, 'cancel'];
         }
 
-        return new Promise(function ($resolve, $reject) use ($promiseOrValue) {
+        return new Promise(function ($resolve, $reject) use ($promiseOrValue): void {
             $promiseOrValue->then($resolve, $reject);
         }, $canceller);
     }
@@ -56,7 +56,7 @@ function resolve($promiseOrValue = null)
  * @param \Throwable $reason
  * @return PromiseInterface
  */
-function reject(\Throwable $reason)
+function reject(\Throwable $reason): PromiseInterface
 {
     return new RejectedPromise($reason);
 }
@@ -70,7 +70,7 @@ function reject(\Throwable $reason)
  * @param array $promisesOrValues
  * @return PromiseInterface
  */
-function all(array $promisesOrValues)
+function all(array $promisesOrValues): PromiseInterface
 {
     return map($promisesOrValues, function ($val) {
         return $val;
@@ -87,15 +87,15 @@ function all(array $promisesOrValues)
  * @param array $promisesOrValues
  * @return PromiseInterface
  */
-function race(array $promisesOrValues)
+function race(array $promisesOrValues): PromiseInterface
 {
     if (!$promisesOrValues) {
-        return new Promise(function () {});
+        return new Promise(function (): void {});
     }
 
     $cancellationQueue = new Internal\CancellationQueue();
 
-    return new Promise(function ($resolve, $reject) use ($promisesOrValues, $cancellationQueue) {
+    return new Promise(function ($resolve, $reject) use ($promisesOrValues, $cancellationQueue): void {
         foreach ($promisesOrValues as $promiseOrValue) {
             $cancellationQueue->enqueue($promiseOrValue);
 
@@ -119,7 +119,7 @@ function race(array $promisesOrValues)
  * @param array $promisesOrValues
  * @return PromiseInterface
  */
-function any(array $promisesOrValues)
+function any(array $promisesOrValues): PromiseInterface
 {
     return some($promisesOrValues, 1)
         ->then(function ($val) {
@@ -145,7 +145,7 @@ function any(array $promisesOrValues)
  * @param int $howMany
  * @return PromiseInterface
  */
-function some(array $promisesOrValues, $howMany)
+function some(array $promisesOrValues, $howMany): PromiseInterface
 {
     if ($howMany < 1) {
         return resolve([]);
@@ -169,14 +169,14 @@ function some(array $promisesOrValues, $howMany)
 
     $cancellationQueue = new Internal\CancellationQueue();
 
-    return new Promise(function ($resolve, $reject) use ($len, $promisesOrValues, $howMany, $cancellationQueue) {
+    return new Promise(function ($resolve, $reject) use ($len, $promisesOrValues, $howMany, $cancellationQueue): void {
         $toResolve = $howMany;
         $toReject  = ($len - $toResolve) + 1;
         $values    = [];
         $reasons   = [];
 
         foreach ($promisesOrValues as $i => $promiseOrValue) {
-            $fulfiller = function ($val) use ($i, &$values, &$toResolve, $toReject, $resolve) {
+            $fulfiller = function ($val) use ($i, &$values, &$toResolve, $toReject, $resolve): void {
                 if ($toResolve < 1 || $toReject < 1) {
                     return;
                 }
@@ -188,7 +188,7 @@ function some(array $promisesOrValues, $howMany)
                 }
             };
 
-            $rejecter = function (\Throwable $reason) use ($i, &$reasons, &$toReject, $toResolve, $reject) {
+            $rejecter = function (\Throwable $reason) use ($i, &$reasons, &$toReject, $toResolve, $reject): void {
                 if ($toResolve < 1 || $toReject < 1) {
                     return;
                 }
@@ -224,7 +224,7 @@ function some(array $promisesOrValues, $howMany)
  * @param callable $mapFunc
  * @return PromiseInterface
  */
-function map(array $promisesOrValues, callable $mapFunc)
+function map(array $promisesOrValues, callable $mapFunc): PromiseInterface
 {
     if (!$promisesOrValues) {
         return resolve([]);
@@ -232,7 +232,7 @@ function map(array $promisesOrValues, callable $mapFunc)
 
     $cancellationQueue = new Internal\CancellationQueue();
 
-    return new Promise(function ($resolve, $reject) use ($promisesOrValues, $mapFunc, $cancellationQueue) {
+    return new Promise(function ($resolve, $reject) use ($promisesOrValues, $mapFunc, $cancellationQueue): void {
         $toResolve = \count($promisesOrValues);
         $values    = [];
 
@@ -243,7 +243,7 @@ function map(array $promisesOrValues, callable $mapFunc)
             resolve($promiseOrValue)
                 ->then($mapFunc)
                 ->done(
-                    function ($mapped) use ($i, &$values, &$toResolve, $resolve) {
+                    function ($mapped) use ($i, &$values, &$toResolve, $resolve): void {
                         $values[$i] = $mapped;
 
                         if (0 === --$toResolve) {
@@ -267,15 +267,15 @@ function ($mapped) use ($i, &$values, &$toResolve, $resolve) {
  * @param mixed $initialValue
  * @return PromiseInterface
  */
-function reduce(array $promisesOrValues, callable $reduceFunc, $initialValue = null)
+function reduce(array $promisesOrValues, callable $reduceFunc, $initialValue = null): PromiseInterface
 {
     $cancellationQueue = new Internal\CancellationQueue();
 
-    return new Promise(function ($resolve, $reject) use ($promisesOrValues, $reduceFunc, $initialValue, $cancellationQueue) {
+    return new Promise(function ($resolve, $reject) use ($promisesOrValues, $reduceFunc, $initialValue, $cancellationQueue): void {
         $total = \count($promisesOrValues);
         $i = 0;
 
-        $wrappedReduceFunc = function ($current, $val) use ($reduceFunc, $cancellationQueue, $total, &$i) {
+        $wrappedReduceFunc = function ($current, $val) use ($reduceFunc, $cancellationQueue, $total, &$i): PromiseInterface {
             $cancellationQueue->enqueue($val);
 
             return $current
@@ -297,7 +297,7 @@ function reduce(array $promisesOrValues, callable $reduceFunc, $initialValue = n
 /**
  * @internal
  */
-function enqueue(callable $task)
+function enqueue(callable $task): void
 {
     static $queue;
 
@@ -311,7 +311,7 @@ function enqueue(callable $task)
 /**
  * @internal
  */
-function fatalError($error)
+function fatalError($error): void
 {
     try {
         \trigger_error($error, E_USER_ERROR);
@@ -324,7 +324,7 @@ function fatalError($error)
 /**
  * @internal
  */
-function _checkTypehint(callable $callback, \Throwable $reason)
+function _checkTypehint(callable $callback, \Throwable $reason): bool
 {
     if (\is_array($callback)) {
         $callbackReflection = new \ReflectionMethod($callback[0], $callback[1]);