diff --git a/tests/CommandTest.php b/tests/CommandTest.php
index 54df15a..06e8742 100644
--- a/tests/CommandTest.php
+++ b/tests/CommandTest.php
@@ -13,7 +13,7 @@
  */
 class CommandTest extends TestCase
 {
-    public function testHasData()
+    public function testHasData(): void
     {
         $c = new Command('foo', ['baz' => 'bar']);
         $this->assertSame('bar', $c['baz']);
@@ -25,20 +25,20 @@ public function testHasData()
         $this->assertInstanceOf('Traversable', $c->getIterator());
     }
 
-    public function testHasParamNormalizesNullToEmptyString()
+    public function testHasParamNormalizesNullToEmptyString(): void
     {
         $c = new Command('foo', ['' => 'bar']);
         $this->assertTrue($c->hasParam(null));
     }
 
-    public function testCanInjectHandlerStack()
+    public function testCanInjectHandlerStack(): void
     {
         $handlerStack = new HandlerStack();
         $c = new Command('foo', [], $handlerStack);
         $this->assertSame($handlerStack, $c->getHandlerStack());
     }
 
-    public function testCloneUsesDifferentHandlerStack()
+    public function testCloneUsesDifferentHandlerStack(): void
     {
         $originalStack = new HandlerStack();
         $command = new Command('foo', [], $originalStack);
diff --git a/tests/Exception/CommandExceptionTest.php b/tests/Exception/CommandExceptionTest.php
index 989bfe5..d3f374c 100644
--- a/tests/Exception/CommandExceptionTest.php
+++ b/tests/Exception/CommandExceptionTest.php
@@ -18,7 +18,7 @@
  */
 class CommandExceptionTest extends TestCase
 {
-    public function testCanGetDataFromException()
+    public function testCanGetDataFromException(): void
     {
         $command = $this->createMock(CommandInterface::class);
         $request = $this->createMock(RequestInterface::class);
@@ -30,7 +30,7 @@ public function testCanGetDataFromException()
         $this->assertSame($response, $exception->getResponse());
     }
 
-    public function testFactoryReturnsExceptionIfAlreadyCommandException()
+    public function testFactoryReturnsExceptionIfAlreadyCommandException(): void
     {
         $command = $this->createMock(CommandInterface::class);
         $previous = CommandException::fromPrevious($command, new \Exception());
@@ -39,7 +39,7 @@ public function testFactoryReturnsExceptionIfAlreadyCommandException()
         $this->assertSame($previous, $exception);
     }
 
-    public function testFactoryReturnsClientExceptionFor400LevelStatusCode()
+    public function testFactoryReturnsClientExceptionFor400LevelStatusCode(): void
     {
         $command = $this->createMock(CommandInterface::class);
         $request = $this->createMock(RequestInterface::class);
@@ -51,7 +51,7 @@ public function testFactoryReturnsClientExceptionFor400LevelStatusCode()
         $this->assertInstanceOf(CommandClientException::class, $exception);
     }
 
-    public function testFactoryReturnsServerExceptionFor500LevelStatusCode()
+    public function testFactoryReturnsServerExceptionFor500LevelStatusCode(): void
     {
         $command = $this->createMock(CommandInterface::class);
         $request = $this->createMock(RequestInterface::class);
diff --git a/tests/ResultTest.php b/tests/ResultTest.php
index 4fdc9b5..1d4a1c0 100644
--- a/tests/ResultTest.php
+++ b/tests/ResultTest.php
@@ -13,7 +13,7 @@
  */
 class ResultTest extends TestCase
 {
-    public function testHasData()
+    public function testHasData(): void
     {
         $c = new Result(['baz' => 'bar']);
         $this->assertSame('bar', $c['baz']);
@@ -27,7 +27,7 @@ public function testHasData()
         $this->assertStringContainsString('bar', (string) $c);
     }
 
-    public function testNullOffsetUsesEmptyStringKey()
+    public function testNullOffsetUsesEmptyStringKey(): void
     {
         $c = new Result(['' => 'bar']);
         $this->assertTrue(isset($c[null]));
@@ -40,7 +40,7 @@ public function testNullOffsetUsesEmptyStringKey()
         $this->assertSame([], $c->toArray());
     }
 
-    public function testAppendSyntaxUsesEmptyStringKey()
+    public function testAppendSyntaxUsesEmptyStringKey(): void
     {
         $c = new Result();
         $c[] = 'bar';
diff --git a/tests/ServiceClientTest.php b/tests/ServiceClientTest.php
index cbcbba6..209a885 100644
--- a/tests/ServiceClientTest.php
+++ b/tests/ServiceClientTest.php
@@ -24,19 +24,19 @@
  */
 class ServiceClientTest extends TestCase
 {
-    private function getServiceClient(array $responses)
+    private function getServiceClient(array $responses): ServiceClient
     {
         return new ServiceClient(
             new HttpClient([
                 'handler' => new MockHandler($responses),
             ]),
-            function (CommandInterface $command) {
+            function (CommandInterface $command): Request {
                 $data = $command->toArray();
                 $data['action'] = $command->getName();
 
                 return new Request('POST', '/', [], http_build_query($data));
             },
-            function (ResponseInterface $response, RequestInterface $request) {
+            function (ResponseInterface $response, RequestInterface $request): Result {
                 $data = json_decode((string) $response->getBody(), true);
                 parse_str((string) $request->getBody(), $data['_request']);
 
@@ -45,17 +45,17 @@ function (ResponseInterface $response, RequestInterface $request) {
         );
     }
 
-    public function testCanGetHttpClientAndHandlers()
+    public function testCanGetHttpClientAndHandlers(): void
     {
         $httpClient = new HttpClient();
         $handlers = new HandlerStack();
-        $fn = function () {};
+        $fn = function (): void {};
         $serviceClient = new ServiceClient($httpClient, $fn, $fn, $handlers);
         $this->assertSame($httpClient, $serviceClient->getHttpClient());
         $this->assertSame($handlers, $serviceClient->getHandlerStack());
     }
 
-    public function testExecuteCommandViaMagicMethod()
+    public function testExecuteCommandViaMagicMethod(): void
     {
         $client = $this->getServiceClient([
             new Response(200, [], '{"foo":"bar"}'),
@@ -74,7 +74,7 @@ public function testExecuteCommandViaMagicMethod()
         $this->assertEquals('doThatThingOtherYouDo', $result2['_request']['action']);
     }
 
-    public function testCommandExceptionIsThrownWhenAnErrorOccurs()
+    public function testCommandExceptionIsThrownWhenAnErrorOccurs(): void
     {
         $client = $this->getServiceClient([
             new BadResponseException(
@@ -88,10 +88,10 @@ public function testCommandExceptionIsThrownWhenAnErrorOccurs()
         $client->execute($client->getCommand('foo'));
     }
 
-    public function testExecuteMultipleCommands()
+    public function testExecuteMultipleCommands(): void
     {
         // Set up commands to execute concurrently.
-        $generateCommands = function () {
+        $generateCommands = function (): \Generator {
             yield new Command('capitalize', ['letter' => 'a']);
             yield new Command('capitalize', ['letter' => '2']);
             yield new Command('capitalize', ['letter' => 'z']);
@@ -113,10 +113,10 @@ public function testExecuteMultipleCommands()
         $fulfilledFnCalled = false;
         $rejectedFnCalled = false;
         $options = [
-            'fulfilled' => function () use (&$fulfilledFnCalled) {
+            'fulfilled' => function () use (&$fulfilledFnCalled): void {
                 $fulfilledFnCalled = true;
             },
-            'rejected' => function () use (&$rejectedFnCalled) {
+            'rejected' => function () use (&$rejectedFnCalled): void {
                 $rejectedFnCalled = true;
             },
         ];
@@ -141,9 +141,9 @@ public function testExecuteMultipleCommands()
         $this->assertEquals('Z', $results[2]['letter']);
     }
 
-    public function testExecuteAllNormalizesNullResultKeys()
+    public function testExecuteAllNormalizesNullResultKeys(): void
     {
-        $generateCommands = function () {
+        $generateCommands = function (): \Generator {
             yield null => new Command('capitalize', ['letter' => 'a']);
         };
 
@@ -153,7 +153,7 @@ public function testExecuteAllNormalizesNullResultKeys()
 
         $fulfilledKey = 'not-called';
         $results = $client->executeAll($generateCommands(), [
-            'fulfilled' => function ($result, $key) use (&$fulfilledKey) {
+            'fulfilled' => function (Result $result, ?string $key) use (&$fulfilledKey): void {
                 $fulfilledKey = $key;
             },
         ]);
@@ -164,9 +164,9 @@ public function testExecuteAllNormalizesNullResultKeys()
         $this->assertSame('A', $results['']['letter']);
     }
 
-    public function testExecuteAllNormalizesNullResultKeysForRejectedCommands()
+    public function testExecuteAllNormalizesNullResultKeysForRejectedCommands(): void
     {
-        $generateCommands = function () {
+        $generateCommands = function (): \Generator {
             yield null => new Command('capitalize', ['letter' => '2']);
         };
 
@@ -180,7 +180,7 @@ public function testExecuteAllNormalizesNullResultKeysForRejectedCommands()
 
         $rejectedKey = 'not-called';
         $results = $client->executeAll($generateCommands(), [
-            'rejected' => function ($reason, $key) use (&$rejectedKey) {
+            'rejected' => function (CommandException $reason, ?string $key) use (&$rejectedKey): void {
                 $rejectedKey = $key;
             },
         ]);
@@ -190,9 +190,9 @@ public function testExecuteAllNormalizesNullResultKeysForRejectedCommands()
         $this->assertInstanceOf(CommandException::class, $results['']);
     }
 
-    public function testMultipleCommandsFailsForNonCommands()
+    public function testMultipleCommandsFailsForNonCommands(): void
     {
-        $generateCommands = function () {
+        $generateCommands = function (): \Generator {
             yield 'foo';
         };