diff --git a/.travis.yml b/.travis.yml
index b84eea2..ad1cb28 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -7,9 +7,6 @@ cache:
         - $HOME/.composer/cache/files
 
 php:
-    - 5.4
-    - 5.5
-    - 5.6
     - 7.0
     - 7.1
     - 7.2
@@ -25,14 +22,11 @@ branches:
 matrix:
     fast_finish: true
     include:
-        - php: 5.4
+        - php: 7.0
           env: COMPOSER_FLAGS="--prefer-stable --prefer-lowest" COVERAGE=true TEST_COMMAND="composer test-ci"
-        - php: hhvm
-          dist: trusty
 
 before_install:
-    - if [[ $COVERAGE != true && $TRAVIS_PHP_VERSION != hhvm ]]; then phpenv config-rm xdebug.ini || true; fi
-    - travis_retry composer self-update
+    - if [[ $COVERAGE != true ]]; then phpenv config-rm xdebug.ini || true; fi
 
 install:
     - travis_retry composer update ${COMPOSER_FLAGS} --prefer-dist --no-interaction
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 42824b4..8ba4c8e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,24 @@
 # Change Log
 
+## 2.0.0 - UNRELEASED
+
+This version is no BC break for consumers using HTTPlug. However, HTTP clients that
+implement HTTPlug need to adjust because we add return type declarations.
+
+### Added
+
+- Support for PSR-18 (HTTP client).
+
+### Removed
+
+- PHP 5 support
+
+### Changed
+
+- [BC Break] `HttpClient::sendRequest(RequestInterface $request)` has a return type annotation. The new 
+signature is `HttpClient::sendRequest(RequestInterface $request): ResponseInterface`.  
+- [BC Break] `RequestException::getRequest()` has a return type annotation. The new 
+signature is `RequestException::getRequest(): RequestInterface`.   
 
 ## 1.1.0 - 2016-08-31
 
diff --git a/composer.json b/composer.json
index 9bf58c1..256dbe0 100644
--- a/composer.json
+++ b/composer.json
@@ -15,8 +15,9 @@
         }
     ],
     "require": {
-        "php": ">=5.4",
+        "php": "^7.0",
         "psr/http-message": "^1.0",
+        "psr/http-client": "^0.1",
         "php-http/promise": "^1.0"
     },
     "require-dev": {
@@ -34,7 +35,7 @@
     },
     "extra": {
         "branch-alias": {
-            "dev-master": "1.2-dev"
+            "dev-master": "2.0-dev"
         }
     }
 }
diff --git a/src/Exception.php b/src/Exception.php
index e7382c3..41e3958 100644
--- a/src/Exception.php
+++ b/src/Exception.php
@@ -2,11 +2,13 @@
 
 namespace Http\Client;
 
+use Psr\Http\Client\ClientException as PsrClientException;
+
 /**
  * Every HTTP Client related Exception must implement this interface.
  *
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  */
-interface Exception
+interface Exception extends PsrClientException
 {
 }
diff --git a/src/Exception/HttpException.php b/src/Exception/HttpException.php
index f4f32a4..5857b01 100644
--- a/src/Exception/HttpException.php
+++ b/src/Exception/HttpException.php
@@ -49,12 +49,6 @@ public function getResponse()
 
     /**
      * Factory method to create a new exception with a normalized error message.
-     *
-     * @param RequestInterface  $request
-     * @param ResponseInterface $response
-     * @param \Exception|null   $previous
-     *
-     * @return HttpException
      */
     public static function create(
         RequestInterface $request,
diff --git a/src/Exception/NetworkException.php b/src/Exception/NetworkException.php
index f2198e5..69b4a4e 100644
--- a/src/Exception/NetworkException.php
+++ b/src/Exception/NetworkException.php
@@ -2,6 +2,8 @@
 
 namespace Http\Client\Exception;
 
+use Psr\Http\Client\Exception\NetworkException as PsrNetworkException;
+
 /**
  * Thrown when the request cannot be completed because of network issues.
  *
@@ -9,6 +11,6 @@
  *
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  */
-class NetworkException extends RequestException
+class NetworkException extends RequestException implements PsrNetworkException
 {
 }
diff --git a/src/Exception/RequestException.php b/src/Exception/RequestException.php
index cdce14b..f95d8d9 100644
--- a/src/Exception/RequestException.php
+++ b/src/Exception/RequestException.php
@@ -3,6 +3,7 @@
 namespace Http\Client\Exception;
 
 use Psr\Http\Message\RequestInterface;
+use Psr\Http\Client\Exception\RequestException as PsrRequestException;
 
 /**
  * Exception for when a request failed, providing access to the failed request.
@@ -12,7 +13,7 @@
  *
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  */
-class RequestException extends TransferException
+class RequestException extends TransferException implements PsrRequestException
 {
     /**
      * @var RequestInterface
@@ -31,12 +32,7 @@ public function __construct($message, RequestInterface $request, \Exception $pre
         parent::__construct($message, 0, $previous);
     }
 
-    /**
-     * Returns the request.
-     *
-     * @return RequestInterface
-     */
-    public function getRequest()
+    public function getRequest(): RequestInterface
     {
         return $this->request;
     }
diff --git a/src/HttpAsyncClient.php b/src/HttpAsyncClient.php
index 492e511..bc9d5ee 100644
--- a/src/HttpAsyncClient.php
+++ b/src/HttpAsyncClient.php
@@ -17,8 +17,6 @@ interface HttpAsyncClient
      *
      * Exceptions related to processing the request are available from the returned Promise.
      *
-     * @param RequestInterface $request
-     *
      * @return Promise Resolves a PSR-7 Response or fails with an Http\Client\Exception.
      *
      * @throws \Exception If processing the request is impossible (eg. bad configuration).
diff --git a/src/HttpClient.php b/src/HttpClient.php
index 0e51749..9dda516 100644
--- a/src/HttpClient.php
+++ b/src/HttpClient.php
@@ -2,6 +2,7 @@
 
 namespace Http\Client;
 
+use Psr\Http\Client\ClientInterface;
 use Psr\Http\Message\RequestInterface;
 use Psr\Http\Message\ResponseInterface;
 
@@ -12,17 +13,13 @@
  * @author Márk Sági-Kazár <mark.sagikazar@gmail.com>
  * @author David Buchmann <mail@davidbu.ch>
  */
-interface HttpClient
+interface HttpClient extends ClientInterface
 {
     /**
      * Sends a PSR-7 request.
      *
-     * @param RequestInterface $request
-     *
-     * @return ResponseInterface
-     *
      * @throws \Http\Client\Exception If an error happens during processing the request.
      * @throws \Exception             If processing the request is impossible (eg. bad configuration).
      */
-    public function sendRequest(RequestInterface $request);
+    public function sendRequest(RequestInterface $request): ResponseInterface;
 }