diff --git a/src/Client.php b/src/Client.php
index f2bb184..ae5cfe3 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -240,7 +240,7 @@ private function createCurlOptions(RequestInterface $request, ResponseBuilder $r
      *
      * @return int
      */
-    private function getProtocolVersion($requestVersion)
+    private function getProtocolVersion(string $requestVersion): int
     {
         switch ($requestVersion) {
             case '1.0':
@@ -265,7 +265,7 @@ private function getProtocolVersion($requestVersion)
      *
      * @return array
      */
-    private function addRequestBodyOptions(RequestInterface $request, array $options)
+    private function addRequestBodyOptions(RequestInterface $request, array $options): array
     {
         /*
          * Some HTTP methods cannot have payload:
@@ -319,7 +319,7 @@ private function addRequestBodyOptions(RequestInterface $request, array $options
      *
      * @return string[]
      */
-    private function createHeaders(RequestInterface $request, array $options)
+    private function createHeaders(RequestInterface $request, array $options): array
     {
         $curlHeaders = [];
         $headers = $request->getHeaders();
@@ -358,7 +358,7 @@ private function createHeaders(RequestInterface $request, array $options)
      *
      * @throws \RuntimeException If creating the stream from $body fails
      */
-    private function createResponseBuilder()
+    private function createResponseBuilder(): ResponseBuilder
     {
         try {
             $body = $this->streamFactory->createStream(fopen('php://temp', 'w+b'));
diff --git a/src/CurlPromise.php b/src/CurlPromise.php
index 5061af5..f04ccf6 100644
--- a/src/CurlPromise.php
+++ b/src/CurlPromise.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl;
 
 use Http\Promise\Promise;
diff --git a/src/MultiRunner.php b/src/MultiRunner.php
index 545f39a..4c60587 100644
--- a/src/MultiRunner.php
+++ b/src/MultiRunner.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl;
 
 use Http\Client\Exception\RequestException;
diff --git a/src/PromiseCore.php b/src/PromiseCore.php
index 09065c4..72a733e 100644
--- a/src/PromiseCore.php
+++ b/src/PromiseCore.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl;
 
 use Http\Client\Exception;
diff --git a/src/ResponseBuilder.php b/src/ResponseBuilder.php
index 805b330..8d98819 100644
--- a/src/ResponseBuilder.php
+++ b/src/ResponseBuilder.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl;
 
 use Http\Message\Builder\ResponseBuilder as OriginalResponseBuilder;
diff --git a/tests/BaseUnitTestCase.php b/tests/BaseUnitTestCase.php
index a51346d..540414c 100644
--- a/tests/BaseUnitTestCase.php
+++ b/tests/BaseUnitTestCase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\PromiseCore;
diff --git a/tests/ClientTest.php b/tests/ClientTest.php
index e21130f..ff6c10b 100644
--- a/tests/ClientTest.php
+++ b/tests/ClientTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\Client;
diff --git a/tests/CurlPromiseTest.php b/tests/CurlPromiseTest.php
index e128e3a..2bab160 100644
--- a/tests/CurlPromiseTest.php
+++ b/tests/CurlPromiseTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\CurlPromise;
diff --git a/tests/HttpAsyncClientDiactorosTest.php b/tests/HttpAsyncClientDiactorosTest.php
index f5b1609..e94cbb0 100644
--- a/tests/HttpAsyncClientDiactorosTest.php
+++ b/tests/HttpAsyncClientDiactorosTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\Client;
diff --git a/tests/HttpAsyncClientGuzzleTest.php b/tests/HttpAsyncClientGuzzleTest.php
index 3d4cb6e..3a9d66e 100644
--- a/tests/HttpAsyncClientGuzzleTest.php
+++ b/tests/HttpAsyncClientGuzzleTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\Client;
diff --git a/tests/HttpAsyncClientTestCase.php b/tests/HttpAsyncClientTestCase.php
index 004becb..8a6f7f0 100644
--- a/tests/HttpAsyncClientTestCase.php
+++ b/tests/HttpAsyncClientTestCase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Tests\HttpAsyncClientTest;
diff --git a/tests/HttpClientDiactorosTest.php b/tests/HttpClientDiactorosTest.php
index 8137ce6..3c33970 100644
--- a/tests/HttpClientDiactorosTest.php
+++ b/tests/HttpClientDiactorosTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\Client;
diff --git a/tests/HttpClientGuzzleTest.php b/tests/HttpClientGuzzleTest.php
index d03fe0a..c3d51cf 100644
--- a/tests/HttpClientGuzzleTest.php
+++ b/tests/HttpClientGuzzleTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use GuzzleHttp\Psr7\Stream;
diff --git a/tests/HttpClientTestCase.php b/tests/HttpClientTestCase.php
index c07274b..cea9e90 100644
--- a/tests/HttpClientTestCase.php
+++ b/tests/HttpClientTestCase.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Tests\HttpClientTest;
diff --git a/tests/PromiseCoreTest.php b/tests/PromiseCoreTest.php
index 9df678b..e3d9d9d 100644
--- a/tests/PromiseCoreTest.php
+++ b/tests/PromiseCoreTest.php
@@ -1,5 +1,7 @@
 <?php
 
+declare(strict_types=1);
+
 namespace Http\Client\Curl\Tests;
 
 use Http\Client\Curl\PromiseCore;