diff --git a/composer.json b/composer.json
index ed7472f..eda9925 100644
--- a/composer.json
+++ b/composer.json
@@ -25,8 +25,8 @@
 		"test": "phpunit"
 	},
 	"require": {
-		"php": "^5.6|^7.0|^8.0"
-	},
+		"php": "^7.1|^8.0"
+    },
 	"require-dev": {
 		"friendsofphp/php-cs-fixer": "3.90.0",
 		"phpunit/phpunit": "^9.6"
diff --git a/src/SumUp/Application/ApplicationConfiguration.php b/src/SumUp/Application/ApplicationConfiguration.php
index 93c0a7c..8c51c03 100644
--- a/src/SumUp/Application/ApplicationConfiguration.php
+++ b/src/SumUp/Application/ApplicationConfiguration.php
@@ -172,7 +172,7 @@ public function __construct(array $config = [])
      *
      * @return string
      */
-    public function getAppId()
+    public function getAppId(): string
     {
         return $this->appId;
     }
@@ -182,7 +182,7 @@ public function getAppId()
      *
      * @return string
      */
-    public function getAppSecret()
+    public function getAppSecret(): string
     {
         return $this->appSecret;
     }
@@ -192,7 +192,7 @@ public function getAppSecret()
      *
      * @return array
      */
-    public function getScopes()
+    public function getScopes(): array
     {
         return $this->scopes;
     }
@@ -202,7 +202,7 @@ public function getScopes()
      *
      * @return string
      */
-    public function getFormattedScopes()
+    public function getFormattedScopes(): string
     {
         return implode(' ', $this->scopes);
     }
@@ -212,7 +212,7 @@ public function getFormattedScopes()
      *
      * @return string
      */
-    public function getBaseURL()
+    public function getBaseURL(): string
     {
         return $this->baseURL;
     }
@@ -220,9 +220,9 @@ public function getBaseURL()
     /**
      * Returns authorization code.
      *
-     * @return null|string
+     * @return string
      */
-    public function getCode()
+    public function getCode(): string
     {
         return $this->code;
     }
@@ -232,7 +232,7 @@ public function getCode()
      *
      * @return string;
      */
-    public function getGrantType()
+    public function getGrantType(): string
     {
         return $this->grantType;
     }
@@ -240,9 +240,9 @@ public function getGrantType()
     /**
      * Returns merchant's username.
      *
-     * @return null|string
+     * @return string
      */
-    public function getUsername()
+    public function getUsername(): string
     {
         return $this->username;
     }
@@ -250,9 +250,9 @@ public function getUsername()
     /**
      * Returns merchant's password.
      *
-     * @return null|string
+     * @return string
      */
-    public function getPassword()
+    public function getPassword(): string
     {
         return $this->password;
     }
@@ -262,7 +262,7 @@ public function getPassword()
      *
      * @return null|string
      */
-    public function getAccessToken()
+    public function getAccessToken(): ?string
     {
         return $this->accessToken;
     }
@@ -272,7 +272,7 @@ public function getAccessToken()
      *
      * @return null|string
      */
-    public function getRefreshToken()
+    public function getRefreshToken(): ?string
     {
         return $this->refreshToken;
     }
@@ -282,7 +282,7 @@ public function getRefreshToken()
      *
      * @return bool
      */
-    public function getForceGuzzle()
+    public function getForceGuzzle(): bool
     {
         return $this->forceGuzzle;
     }
@@ -292,7 +292,7 @@ public function getForceGuzzle()
      *
      * @return array
      */
-    public function getCustomHeaders()
+    public function getCustomHeaders(): array
     {
         return $this->customHeaders;
     }
@@ -302,7 +302,7 @@ public function getCustomHeaders()
      *
      * @return string|null
      */
-    public function getCABundlePath()
+    public function getCABundlePath(): ?string
     {
         if (!empty($this->caBundlePath)) {
             return $this->caBundlePath;
@@ -316,7 +316,7 @@ public function getCABundlePath()
      *
      * @return string|null
      */
-    public function getApiKey()
+    public function getApiKey(): ?string
     {
         return $this->apiKey;
     }
@@ -324,11 +324,11 @@ public function getApiKey()
     /**
      * Set application ID.
      *
-     * @param string $appId
+     * @param ?string $appId
      *
      * @throws SumUpConfigurationException
      */
-    protected function setAppId($appId)
+    protected function setAppId(?string $appId): void
     {
         if (empty($appId) && empty($this->apiKey)) {
             throw new SumUpConfigurationException('Missing mandatory parameter app_id or api_key');
@@ -343,7 +343,7 @@ protected function setAppId($appId)
      *
      * @throws SumUpConfigurationException
      */
-    protected function setAppSecret($appSecret)
+    protected function setAppSecret(?string $appSecret): void
     {
         if (empty($appSecret) && empty($this->apiKey)) {
             throw new SumUpConfigurationException('Missing mandatory parameter app_secret or api_key');
@@ -354,11 +354,11 @@ protected function setAppSecret($appSecret)
     /**
      * Set the authorization grant type.
      *
-     * @param array $grantType
+     * @param string $grantType
      *
      * @throws SumUpConfigurationException
      */
-    protected function setGrantType($grantType)
+    protected function setGrantType(string $grantType): void
     {
         if (!in_array($grantType, $this::GRANT_TYPES)) {
             throw new SumUpConfigurationException('Invalid parameter for "grant_type". Allowed values are: ' . implode(' | ', $this::GRANT_TYPES) . '.');
@@ -371,24 +371,18 @@ protected function setGrantType($grantType)
      *
      * @param array $scopes
      */
-    protected function setScopes(array $scopes = [])
+    protected function setScopes(array $scopes = []): void
     {
         $this->scopes = array_unique(array_merge($this::DEFAULT_SCOPES, $scopes), SORT_REGULAR);
-        ;
     }
 
     /**
      * Set the flag whether to use GuzzleHttp.
      *
      * @param bool $forceGuzzle
-     *
-     * @throws SumUpConfigurationException
      */
-    protected function setForceGuzzle($forceGuzzle)
+    protected function setForceGuzzle(bool $forceGuzzle): void
     {
-        if (!is_bool($forceGuzzle)) {
-            throw new SumUpConfigurationException('Invalid value for boolean parameter use_guzzlehttp_over_curl.');
-        }
         $this->forceGuzzle = $forceGuzzle;
     }
 
@@ -397,12 +391,11 @@ protected function setForceGuzzle($forceGuzzle)
      *
      * @param array $customHeaders
      */
-    public function setCustomHeaders($customHeaders)
+    public function setCustomHeaders(array $customHeaders): void
     {
-        $headers = is_array($customHeaders) ? $customHeaders : [];
-        $headers[self::USER_AGENT_HEADER] = SdkInfo::getUserAgent();
+        $customHeaders[self::USER_AGENT_HEADER] = SdkInfo::getUserAgent();
 
-        $this->customHeaders = $headers;
+        $this->customHeaders = $customHeaders;
     }
 
     /**
@@ -412,17 +405,13 @@ public function setCustomHeaders($customHeaders)
      *
      * @throws SumUpConfigurationException
      */
-    protected function setCABundlePath($caBundlePath)
+    protected function setCABundlePath(?string $caBundlePath): void
     {
         if ($caBundlePath === null || $caBundlePath === '') {
             $this->caBundlePath = null;
             return;
         }
 
-        if (!is_string($caBundlePath)) {
-            throw new SumUpConfigurationException('Invalid value for "ca_bundle_path". Expected string path or null.');
-        }
-
         if (!is_readable($caBundlePath)) {
             throw new SumUpConfigurationException(sprintf('The provided ca_bundle_path "%s" is not readable.', $caBundlePath));
         }
@@ -435,7 +424,7 @@ protected function setCABundlePath($caBundlePath)
      *
      * @return string|null
      */
-    private function getDefaultCABundlePath()
+    private function getDefaultCABundlePath(): ?string
     {
         $path = realpath(__DIR__ . '/../../../resources/ca-bundle.crt');
         if ($path && is_readable($path)) {
diff --git a/src/SumUp/Application/ApplicationConfigurationInterface.php b/src/SumUp/Application/ApplicationConfigurationInterface.php
index 755d213..a855580 100644
--- a/src/SumUp/Application/ApplicationConfigurationInterface.php
+++ b/src/SumUp/Application/ApplicationConfigurationInterface.php
@@ -14,96 +14,96 @@ interface ApplicationConfigurationInterface
      *
      * @return string
      */
-    public function getAppId();
+    public function getAppId(): string;
 
     /**
      * Returns application's secret.
      *
      * @return string
      */
-    public function getAppSecret();
+    public function getAppSecret(): string;
 
     /**
      * Returns the scopes formatted as they should appear in the request.
      *
-     * @return string
+     * @return array
      */
-    public function getScopes();
+    public function getScopes(): array;
 
     /**
      * Returns the base URL of the SumUp API.
      *
      * @return string
      */
-    public function getBaseURL();
+    public function getBaseURL(): string;
 
     /**
      * Returns authorization code.
      *
      * @return string
      */
-    public function getCode();
+    public function getCode(): string;
 
     /**
      * Returns grant type.
      *
      * @return string
      */
-    public function getGrantType();
+    public function getGrantType(): string;
 
     /**
      * Returns merchant's username;
      *
      * @return string
      */
-    public function getUsername();
+    public function getUsername(): string;
 
     /**
-     * Returns merchant's passowrd;
+     * Returns merchant's password;
      *
      * @return string
      */
-    public function getPassword();
+    public function getPassword(): string;
 
     /**
      * Returns access token.
      *
-     * @return string
+     * @return string|null
      */
-    public function getAccessToken();
+    public function getAccessToken(): ?string;
 
     /**
      * Returns refresh token.
      *
      * @return string
      */
-    public function getRefreshToken();
+    public function getRefreshToken(): ?string;
 
     /**
      * Returns a flag whether to use GuzzleHttp over cURL if both are present.
      *
      * @return bool
      */
-    public function getForceGuzzle();
+    public function getForceGuzzle(): bool;
 
     /**
      * Returns associative array with custom headers.
      *
      * @return array
      */
-    public function getCustomHeaders();
+    public function getCustomHeaders(): array;
 
     /**
      * Returns the path to the CA bundle used for HTTPS verification.
      *
      * @return string|null
      */
-    public function getCABundlePath();
+    public function getCABundlePath(): ?string;
 
     /**
      * Returns the API key if set.
      *
      * @return string|null
      */
-    public function getApiKey();
+    public function getApiKey(): ?string;
 }
diff --git a/src/SumUp/Authentication/AccessToken.php b/src/SumUp/Authentication/AccessToken.php
index 413b85b..90dd869 100644
--- a/src/SumUp/Authentication/AccessToken.php
+++ b/src/SumUp/Authentication/AccessToken.php
@@ -49,11 +49,11 @@ class AccessToken
      *
      * @param string $value
      * @param string $type
-     * @param int    $expiresIn
+     * @param int|null    $expiresIn
      * @param array  $scope
-     * @param string $refreshToken
+     * @param string|null $refreshToken
      */
-    public function __construct($value, $type = '', $expiresIn = -1, array $scope = [], $refreshToken = null)
+    public function __construct(string $value, string $type = '', ?int $expiresIn = -1, array $scope = [], ?string $refreshToken = null)
     {
         if ($value) {
             $this->value = $value;
@@ -77,7 +77,7 @@ public function __construct($value, $type = '', $expiresIn = -1, array $scope =
      *
      * @return string
      */
-    public function getValue()
+    public function getValue(): string
     {
         return $this->value;
     }
@@ -87,7 +87,7 @@ public function getValue()
      *
      * @return string
      */
-    public function getType()
+    public function getType(): string
     {
         return $this->type;
     }
@@ -97,7 +97,7 @@ public function getType()
      *
      * @return int
      */
-    public function getExpiresIn()
+    public function getExpiresIn(): int
     {
         return $this->expiresIn;
     }
@@ -107,7 +107,7 @@ public function getExpiresIn()
      *
      * @return array
      */
-    public function getScopes()
+    public function getScopes(): array
     {
         return $this->scope;
     }
@@ -117,7 +117,7 @@ public function getScopes()
      *
      * @return null|string
      */
-    public function getRefreshToken()
+    public function getRefreshToken(): ?string
     {
         return $this->refreshToken;
     }
diff --git a/src/SumUp/Exceptions/SumUpValidationException.php b/src/SumUp/Exceptions/SumUpValidationException.php
index fb99adf..1d12618 100644
--- a/src/SumUp/Exceptions/SumUpValidationException.php
+++ b/src/SumUp/Exceptions/SumUpValidationException.php
@@ -2,6 +2,8 @@
 
 namespace SumUp\Exceptions;
 
+use Throwable;
+
 /**
  * Class SumUpValidationException
  *
@@ -22,9 +24,9 @@ class SumUpValidationException extends SumUpSDKException
      *
      * @param array $fields
      * @param int   $code
-     * @param null  $previous
+     * @param Throwable|null  $previous
      */
-    public function __construct($fields = [], $code = 0, $previous = null)
+    public function __construct(array $fields = [], int $code = 0, ?Throwable $previous = null)
     {
         $this->fields = $fields;
         $message = self::VALIDATION_ERROR_BASE . implode(', ', $fields);
@@ -36,7 +38,7 @@ public function __construct($fields = [], $code = 0, $previous = null)
      *
      * @return array
      */
-    public function getInvalidFields()
+    public function getInvalidFields(): array
     {
         return $this->fields;
     }
diff --git a/src/SumUp/HttpClients/HttpClientsFactory.php b/src/SumUp/HttpClients/HttpClientsFactory.php
index 61f1667..63b4893 100644
--- a/src/SumUp/HttpClients/HttpClientsFactory.php
+++ b/src/SumUp/HttpClients/HttpClientsFactory.php
@@ -27,7 +27,7 @@ private function __construct()
      *
      * @throws SumUpConfigurationException
      */
-    public static function createHttpClient(ApplicationConfigurationInterface $appConfig, SumUpHttpClientInterface $customHttpClient = null)
+    public static function createHttpClient(ApplicationConfigurationInterface $appConfig, ?SumUpHttpClientInterface $customHttpClient = null): SumUpHttpClientInterface
     {
         if ($customHttpClient) {
             return $customHttpClient;
@@ -45,12 +45,14 @@ public static function createHttpClient(ApplicationConfigurationInterface $appCo
      *
      * @param string $baseURL
      * @param bool $forceUseGuzzle
+     * @param array $customHeaders
+     * @param string|null $caBundlePath
      *
      * @return SumUpCUrlClient|SumUpGuzzleHttpClient
      *
      * @throws SumUpConfigurationException
      */
-    private static function detectDefaultClient($baseURL, $forceUseGuzzle, $customHeaders, $caBundlePath)
+    private static function detectDefaultClient(string $baseURL, bool $forceUseGuzzle, array $customHeaders, ?string $caBundlePath): SumUpHttpClientInterface
     {
         if (extension_loaded('curl') && !$forceUseGuzzle) {
             return new SumUpCUrlClient($baseURL, $customHeaders, $caBundlePath);
diff --git a/src/SumUp/HttpClients/Response.php b/src/SumUp/HttpClients/Response.php
index 61aba4a..1a8cd13 100644
--- a/src/SumUp/HttpClients/Response.php
+++ b/src/SumUp/HttpClients/Response.php
@@ -4,6 +4,7 @@
 
 use SumUp\Exceptions\SumUpAuthenticationException;
 use SumUp\Exceptions\SumUpResponseException;
+use SumUp\Exceptions\SumUpSDKException;
 use SumUp\Exceptions\SumUpServerException;
 use SumUp\Exceptions\SumUpValidationException;
 
@@ -17,7 +18,7 @@ class Response
     /**
      * The HTTP response code.
      *
-     * @var number
+     * @var int
      */
     protected $httpResponseCode;
 
@@ -31,17 +32,17 @@ class Response
     /**
      * Response constructor.
      *
-     * @param number $httpResponseCode
-     * @param $body
+     * @param int $httpResponseCode
+     * @param mixed $body
      *
      * @throws SumUpAuthenticationException
      * @throws SumUpResponseException
      * @throws SumUpValidationException
      * @throws SumUpServerException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      * @throws SumUpValidationException
      */
-    public function __construct($httpResponseCode, $body)
+    public function __construct(int $httpResponseCode, $body)
     {
         $this->httpResponseCode = $httpResponseCode;
         $this->body = $body;
@@ -51,9 +52,9 @@ public function __construct($httpResponseCode, $body)
     /**
      * Get HTTP response code.
      *
-     * @return number
+     * @return int
      */
-    public function getHttpResponseCode()
+    public function getHttpResponseCode(): int
     {
         return $this->httpResponseCode;
     }
@@ -77,9 +78,9 @@ public function getBody()
      * @throws SumUpResponseException
      * @throws SumUpValidationException
      * @throws SumUpServerException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    protected function parseResponseForErrors()
+    protected function parseResponseForErrors(): void
     {
         if (isset($this->body->error_code) && $this->body->error_code === 'NOT_AUTHORIZED') {
             throw new SumUpAuthenticationException($this->body->error_message, $this->httpResponseCode);
@@ -111,7 +112,7 @@ protected function parseResponseForErrors()
      *
      * @return string
      */
-    protected function parseErrorMessage($defaultMessage = '')
+    protected function parseErrorMessage(string $defaultMessage = ''): string
     {
         if (is_null($this->body)) {
             return $defaultMessage;
diff --git a/src/SumUp/HttpClients/SumUpCUrlClient.php b/src/SumUp/HttpClients/SumUpCUrlClient.php
index 652c4b7..1241856 100644
--- a/src/SumUp/HttpClients/SumUpCUrlClient.php
+++ b/src/SumUp/HttpClients/SumUpCUrlClient.php
@@ -2,8 +2,11 @@
 
 namespace SumUp\HttpClients;
 
+use SumUp\Exceptions\SumUpAuthenticationException;
 use SumUp\Exceptions\SumUpConnectionException;
+use SumUp\Exceptions\SumUpResponseException;
 use SumUp\Exceptions\SumUpSDKException;
+use SumUp\Exceptions\SumUpValidationException;
 
 /**
  * Class SumUpCUrlClient
@@ -40,7 +43,7 @@ class SumUpCUrlClient implements SumUpHttpClientInterface
      * @param array       $customHeaders
      * @param string|null $caBundlePath
      */
-    public function __construct($baseUrl, $customHeaders, $caBundlePath = null)
+    public function __construct(string $baseUrl, array $customHeaders, ?string $caBundlePath = null)
     {
         $this->baseUrl = $baseUrl;
         $this->customHeaders = $customHeaders;
@@ -56,12 +59,12 @@ public function __construct($baseUrl, $customHeaders, $caBundlePath = null)
      * @return Response
      *
      * @throws SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpValidationException
+     * @throws SumUpResponseException
+     * @throws SumUpAuthenticationException
+     * @throws SumUpValidationException
      * @throws SumUpSDKException
      */
-    public function send($method, $url, $body, $headers = [])
+    public function send(string $method, string $url, array $body, array $headers = []): Response
     {
         $reqHeaders = array_merge($headers, $this->customHeaders);
         $ch = curl_init();
@@ -98,7 +101,7 @@ public function send($method, $url, $body, $headers = [])
      *
      * @return array
      */
-    private function formatHeaders($headers = null)
+    private function formatHeaders(?array $headers = null): array
     {
         if (count($headers) == 0) {
             return $headers;
@@ -115,9 +118,9 @@ private function formatHeaders($headers = null)
     /**
      * Returns JSON encoded the response's body if it is of JSON type.
      *
-     * @param $response
+     * @param bool|string $response
      *
-     * @return mixed
+     * @return bool|string
      */
     private function parseBody($response)
     {
diff --git a/src/SumUp/HttpClients/SumUpGuzzleHttpClient.php b/src/SumUp/HttpClients/SumUpGuzzleHttpClient.php
index fe70db1..c5ddd44 100644
--- a/src/SumUp/HttpClients/SumUpGuzzleHttpClient.php
+++ b/src/SumUp/HttpClients/SumUpGuzzleHttpClient.php
@@ -2,14 +2,19 @@
 
 namespace SumUp\HttpClients;
 
+use Exception;
 use GuzzleHttp\Client;
 use GuzzleHttp\Exception\ClientException;
 use GuzzleHttp\Exception\ConnectException;
+use GuzzleHttp\Exception\GuzzleException;
 use GuzzleHttp\Exception\ServerException;
 use GuzzleHttp\Psr7\Request;
+use SumUp\Exceptions\SumUpAuthenticationException;
 use SumUp\Exceptions\SumUpConnectionException;
+use SumUp\Exceptions\SumUpResponseException;
 use SumUp\Exceptions\SumUpSDKException;
 use SumUp\Exceptions\SumUpServerException;
+use SumUp\Exceptions\SumUpValidationException;
 
 /**
  * Class SumUpGuzzleHttpClient
@@ -21,14 +26,14 @@ class SumUpGuzzleHttpClient implements SumUpHttpClientInterface
     /**
      * The Guzzle Client instance.
      *
-     * @var client
+     * @var Client $guzzleClient
      */
     private $guzzleClient;
 
     /**
      * Custom headers for every request.
      *
-     * @var $customHeaders
+     * @var array $customHeaders
      */
     private $customHeaders;
 
@@ -39,7 +44,7 @@ class SumUpGuzzleHttpClient implements SumUpHttpClientInterface
      * @param array       $customHeaders
      * @param string|bool|null $caBundlePath
      */
-    public function __construct($baseUrl, $customHeaders, $caBundlePath = null)
+    public function __construct(string $baseUrl, array $customHeaders, $caBundlePath = null)
     {
         $options = ['base_uri' => $baseUrl];
         if ($caBundlePath !== null) {
@@ -60,12 +65,12 @@ public function __construct($baseUrl, $customHeaders, $caBundlePath = null)
      *
      * @throws SumUpConnectionException
      * @throws SumUpServerException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpValidationException
+     * @throws SumUpResponseException
+     * @throws SumUpAuthenticationException
+     * @throws SumUpValidationException
      * @throws SumUpSDKException
      */
-    public function send($method, $url, $body, $headers = [])
+    public function send(string $method, string $url, array $body, array $headers = []): Response
     {
         $options = [
             'headers' => array_merge($headers, $this->customHeaders),
@@ -85,15 +90,9 @@ public function send($method, $url, $body, $headers = [])
         } catch (ServerException $e) {
             $response = $e->getResponse();
             $body = $this->parseBody($response);
-            if (isset($body) && isset($body->message)) {
-                $message = $body->message;
-            } else {
-                $message = $body;
-            }
+            $message = $body->message ?? $body;
             throw new SumUpServerException($message, $e->getCode(), $e->getPrevious());
-        } catch (\GuzzleHttp\Exception\GuzzleException $e) {
-            throw new SumUpSDKException($e->getMessage(), $e->getCode(), $e->getPrevious());
-        } catch (\Exception $e) {
+        } catch (GuzzleException|Exception $e) {
             throw new SumUpSDKException($e->getMessage(), $e->getCode(), $e->getPrevious());
         }
         $body = $this->parseBody($response);
diff --git a/src/SumUp/HttpClients/SumUpHttpClientInterface.php b/src/SumUp/HttpClients/SumUpHttpClientInterface.php
index 8f726d1..072ef25 100644
--- a/src/SumUp/HttpClients/SumUpHttpClientInterface.php
+++ b/src/SumUp/HttpClients/SumUpHttpClientInterface.php
@@ -17,5 +17,5 @@ interface SumUpHttpClientInterface
      *
      * @return Response|mixed
      */
-    public function send($method, $url, $body, $headers);
+    public function send(string $method, string $url, array $body, array $headers);
 }
diff --git a/src/SumUp/SdkInfo.php b/src/SumUp/SdkInfo.php
index 1002a9f..1e67b1f 100644
--- a/src/SumUp/SdkInfo.php
+++ b/src/SumUp/SdkInfo.php
@@ -14,7 +14,7 @@ class SdkInfo
      *
      * @return string
      */
-    public static function getVersion()
+    public static function getVersion(): string
     {
         return Version::CURRENT;
     }
@@ -24,7 +24,7 @@ public static function getVersion()
      *
      * @return string
      */
-    public static function getUserAgent()
+    public static function getUserAgent(): string
     {
         return sprintf(self::USER_AGENT_TEMPLATE, self::getVersion());
     }
diff --git a/src/SumUp/Services/Authorization.php b/src/SumUp/Services/Authorization.php
index e6d5053..84c58c8 100644
--- a/src/SumUp/Services/Authorization.php
+++ b/src/SumUp/Services/Authorization.php
@@ -3,8 +3,12 @@
 namespace SumUp\Services;
 
 use SumUp\Application\ApplicationConfigurationInterface;
+use SumUp\Exceptions\SumUpAuthenticationException;
 use SumUp\Exceptions\SumUpConfigurationException;
 use SumUp\Exceptions\SumUpArgumentException;
+use SumUp\Exceptions\SumUpConnectionException;
+use SumUp\Exceptions\SumUpResponseException;
+use SumUp\Exceptions\SumUpSDKException;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Application\ApplicationConfiguration;
 use SumUp\Authentication\AccessToken;
@@ -50,12 +54,12 @@ public function __construct(SumUpHttpClientInterface $client, ApplicationConfigu
      * @return null|AccessToken
      *
      * @throws SumUpConfigurationException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpAuthenticationException
+     * @throws SumUpConnectionException
+     * @throws SumUpResponseException
+     * @throws SumUpSDKException
      */
-    public function getToken()
+    public function getToken(): ?AccessToken
     {
         if (!empty($this->appConfig->getApiKey())) {
             return new AccessToken(
@@ -104,13 +108,8 @@ public function getToken()
      * Returns an access token for the grant type "authorization_code".
      *
      * @return AccessToken
-     *
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
      */
-    public function getTokenByCode()
+    public function getTokenByCode(): AccessToken
     {
         $payload = [
             'grant_type' => 'authorization_code',
@@ -133,13 +132,8 @@ public function getTokenByCode()
      * Returns an access token for the grant type "client_credentials".
      *
      * @return AccessToken
-     *
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
      */
-    public function getTokenByClientCredentials()
+    public function getTokenByClientCredentials(): AccessToken
     {
         $payload = [
             'grant_type' => 'client_credentials',
@@ -159,12 +153,9 @@ public function getTokenByClientCredentials()
      * @return AccessToken
      *
      * @throws SumUpConfigurationException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function getTokenByPassword()
+    public function getTokenByPassword(): AccessToken
     {
         if (empty($this->appConfig->getUsername())) {
             throw new SumUpConfigurationException(ExceptionMessages::getMissingParamMsg('username'));
@@ -198,12 +189,9 @@ public function getTokenByPassword()
      * @return AccessToken
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function refreshToken($refreshToken)
+    public function refreshToken(string $refreshToken): AccessToken
     {
         if (empty($refreshToken)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('refresh token'));
diff --git a/src/SumUp/Services/Checkouts.php b/src/SumUp/Services/Checkouts.php
index 6dc446f..1cb5bbb 100644
--- a/src/SumUp/Services/Checkouts.php
+++ b/src/SumUp/Services/Checkouts.php
@@ -3,6 +3,8 @@
 namespace SumUp\Services;
 
 use SumUp\Exceptions\SumUpArgumentException;
+use SumUp\Exceptions\SumUpSDKException;
+use SumUp\HttpClients\Response;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Authentication\AccessToken;
 use SumUp\Utils\ExceptionMessages;
@@ -49,20 +51,25 @@ public function __construct(SumUpHttpClientInterface $client, AccessToken $acces
      * @param string $checkoutRef
      * @param string $merchantCode
      * @param string $description
-     * @param null   $payFromEmail
-     * @param null   $returnURL
-     * @param null   $redirectURL
+     * @param null|string   $payFromEmail
+     * @param null|string   $returnURL
+     * @param null|string   $redirectURL
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function create($amount, $currency, $checkoutRef, $merchantCode, $description = '', $payFromEmail = null, $returnURL = null, $redirectURL = null)
-    {
+    public function create(
+        float $amount,
+        string $currency,
+        string $checkoutRef,
+        string $merchantCode,
+        string $description = '',
+        ?string $payFromEmail = null,
+        ?string $returnURL = null,
+        ?string $redirectURL = null
+    ): Response {
         if (empty($amount) || !is_numeric($amount)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('amount'));
         }
@@ -103,15 +110,12 @@ public function create($amount, $currency, $checkoutRef, $merchantCode, $descrip
      *
      * @param string $checkoutId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function findById($checkoutId)
+    public function findById(string $checkoutId): Response
     {
         if (empty($checkoutId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('checkout id'));
@@ -126,15 +130,12 @@ public function findById($checkoutId)
      *
      * @param string $referenceId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function findByReferenceId($referenceId)
+    public function findByReferenceId(string $referenceId): Response
     {
         if (empty($referenceId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('reference id'));
@@ -149,15 +150,12 @@ public function findByReferenceId($referenceId)
      *
      * @param string $checkoutId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function delete($checkoutId)
+    public function delete(string $checkoutId): Response
     {
         if (empty($checkoutId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('checkout id'));
@@ -175,15 +173,12 @@ public function delete($checkoutId)
      * @param string $cardToken
      * @param int    $installments
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function pay($checkoutId, $customerId, $cardToken, $installments = 1)
+    public function pay(string $checkoutId, string $customerId, string $cardToken, int $installments = 1): Response
     {
         if (empty($checkoutId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('checkout id'));
diff --git a/src/SumUp/Services/Custom.php b/src/SumUp/Services/Custom.php
index 356d289..6ea448e 100644
--- a/src/SumUp/Services/Custom.php
+++ b/src/SumUp/Services/Custom.php
@@ -3,6 +3,8 @@
 namespace SumUp\Services;
 
 use SumUp\Exceptions\SumUpArgumentException;
+use SumUp\Exceptions\SumUpSDKException;
+use SumUp\HttpClients\Response;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Authentication\AccessToken;
 use SumUp\Utils\Headers;
@@ -52,15 +54,12 @@ public function __construct(SumUpHttpClientInterface $client, AccessToken $acces
      * @param string     $relativePath
      * @param array|null $payload
      *
-     * @return mixed|\SumUp\HttpClients\Response
+     * @return mixed|Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function request($method, $relativePath, $payload = null)
+    public function request(string $method, string $relativePath, ?array $payload = null)
     {
         if (!in_array($method, $this::HTTP_METHODS)) {
             $message = "Not allowed method provided: $method. Allowed values: " . implode(', ', $this::HTTP_METHODS) . '.';
diff --git a/src/SumUp/Services/Customers.php b/src/SumUp/Services/Customers.php
index e01b40a..fa2c301 100644
--- a/src/SumUp/Services/Customers.php
+++ b/src/SumUp/Services/Customers.php
@@ -2,6 +2,8 @@
 
 namespace SumUp\Services;
 
+use SumUp\Exceptions\SumUpSDKException;
+use SumUp\HttpClients\Response;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Authentication\AccessToken;
 use SumUp\Exceptions\SumUpArgumentException;
@@ -44,19 +46,16 @@ public function __construct(SumUpHttpClientInterface $client, AccessToken $acces
     /**
      * Create new customer.
      *
-     * @param $customerId
+     * @param string $customerId
      * @param array $customerDetails
      * @param array $customerAddress
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function create($customerId, array $customerDetails = [], array $customerAddress = [])
+    public function create(string $customerId, array $customerDetails = [], array $customerAddress = []): Response
     {
         if (empty($customerId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('customer id'));
@@ -96,19 +95,16 @@ public function create($customerId, array $customerDetails = [], array $customer
     /**
      * Update existing customer.
      *
-     * @param $customerId
+     * @param string $customerId
      * @param array $customerDetails
      * @param array $customerAddress
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function update($customerId, array $customerDetails = [], array $customerAddress = [])
+    public function update(string $customerId, array $customerDetails = [], array $customerAddress = []): Response
     {
         if (empty($customerId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('customer id'));
@@ -147,17 +143,14 @@ public function update($customerId, array $customerDetails = [], array $customer
     /**
      * Get customer by ID.
      *
-     * @param $customerId
+     * @param string $customerId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function get($customerId)
+    public function get(string $customerId): Response
     {
         if (empty($customerId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('customer id'));
@@ -170,17 +163,14 @@ public function get($customerId)
     /**
      * Get payment instruments for a customer.
      *
-     * @param $customerId
+     * @param string $customerId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function getPaymentInstruments($customerId)
+    public function getPaymentInstruments(string $customerId): Response
     {
         if (empty($customerId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('customer id'));
@@ -193,18 +183,15 @@ public function getPaymentInstruments($customerId)
     /**
      * Deactivate payment instrument for a customer.
      *
-     * @param $customerId
-     * @param $cardToken
+     * @param string $customerId
+     * @param string $cardToken
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function deletePaymentInstruments($customerId, $cardToken)
+    public function deletePaymentInstruments(string $customerId, string $cardToken): Response
     {
         if (empty($customerId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('customer id'));
diff --git a/src/SumUp/Services/Merchant.php b/src/SumUp/Services/Merchant.php
index dcf3c36..6a1a547 100644
--- a/src/SumUp/Services/Merchant.php
+++ b/src/SumUp/Services/Merchant.php
@@ -2,6 +2,8 @@
 
 namespace SumUp\Services;
 
+use SumUp\Exceptions\SumUpSDKException;
+use SumUp\HttpClients\Response;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Authentication\AccessToken;
 use SumUp\Exceptions\SumUpArgumentException;
@@ -44,14 +46,9 @@ public function __construct(SumUpHttpClientInterface $client, AccessToken $acces
     /**
      * Get merchant's profile.
      *
-     * @return \SumUp\HttpClients\Response
-     *
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @return Response
      */
-    public function getProfile()
+    public function getProfile(): Response
     {
         $path = '/v0.1/me/merchant-profile';
         $headers = array_merge(Headers::getStandardHeaders(), Headers::getAuth($this->accessToken));
@@ -63,15 +60,12 @@ public function getProfile()
      *
      * @param array $data
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function updateProfile(array $data)
+    public function updateProfile(array $data): Response
     {
         if (empty($data)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('payload data'));
@@ -84,14 +78,9 @@ public function updateProfile(array $data)
     /**
      * Get data for doing business as.
      *
-     * @return \SumUp\HttpClients\Response
-     *
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @return Response
      */
-    public function getDoingBusinessAs()
+    public function getDoingBusinessAs(): Response
     {
         $path = '/v0.1/me/merchant-profile/doing-business-as';
         $headers = array_merge(Headers::getStandardHeaders(), Headers::getAuth($this->accessToken));
@@ -103,15 +92,12 @@ public function getDoingBusinessAs()
      *
      * @param array $data
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function updateDoingBusinessAs(array $data)
+    public function updateDoingBusinessAs(array $data): Response
     {
         if (empty($data)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('payload data'));
diff --git a/src/SumUp/Services/Payouts.php b/src/SumUp/Services/Payouts.php
index 2e100a5..df542ec 100644
--- a/src/SumUp/Services/Payouts.php
+++ b/src/SumUp/Services/Payouts.php
@@ -2,6 +2,8 @@
 
 namespace SumUp\Services;
 
+use SumUp\Exceptions\SumUpSDKException;
+use SumUp\HttpClients\Response;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Authentication\AccessToken;
 use SumUp\Exceptions\SumUpArgumentException;
@@ -50,15 +52,12 @@ public function __construct(SumUpHttpClientInterface $client, AccessToken $acces
      * @param bool   $descendingOrder
      * @param string $format
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function getPayouts($startDate, $endDate, $limit = 10, $descendingOrder = true, $format = 'json')
+    public function getPayouts(string $startDate, string $endDate, int $limit = 10, bool $descendingOrder = true, string $format = 'json'): Response
     {
         if (empty($startDate)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('start date'));
@@ -69,9 +68,6 @@ public function getPayouts($startDate, $endDate, $limit = 10, $descendingOrder =
         if (empty($limit) || !is_int($limit)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('limit'));
         }
-        if (empty($descendingOrder)) {
-            throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('order'));
-        }
         if (empty($format)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('format'));
         }
@@ -85,11 +81,11 @@ public function getPayouts($startDate, $endDate, $limit = 10, $descendingOrder =
         $queryParams = http_build_query($filters);
         $path = '/v0.1/me/financials/payouts?' . $queryParams;
         $headers = array_merge(Headers::getStandardHeaders(), Headers::getAuth($this->accessToken));
-        return $this->client->send('GET', $path, null, $headers);
+        return $this->client->send('GET', $path, [], $headers);
     }
 
     /**
-     * Get a list of payed out transactions.
+     * Get a list of paid out transactions.
      *
      * @param string $startDate
      * @param string $endDate
@@ -97,15 +93,12 @@ public function getPayouts($startDate, $endDate, $limit = 10, $descendingOrder =
      * @param bool   $descendingOrder
      * @param string $format
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function getTransactions($startDate, $endDate, $limit = 10, $descendingOrder = true, $format = 'json')
+    public function getTransactions(string $startDate, string $endDate, int $limit = 10, bool $descendingOrder = true, string $format = 'json'): Response
     {
         if (empty($startDate)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('start date'));
@@ -116,9 +109,6 @@ public function getTransactions($startDate, $endDate, $limit = 10, $descendingOr
         if (empty($limit) || !is_int($limit)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('limit'));
         }
-        if (empty($descendingOrder)) {
-            throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('order'));
-        }
         if (empty($format)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('format'));
         }
@@ -132,6 +122,6 @@ public function getTransactions($startDate, $endDate, $limit = 10, $descendingOr
         $queryParams = http_build_query($filters);
         $path = '/v0.1/me/financials/transactions?' . $queryParams;
         $headers = array_merge(Headers::getStandardHeaders(), Headers::getAuth($this->accessToken));
-        return $this->client->send('GET', $path, null, $headers);
+        return $this->client->send('GET', $path, [], $headers);
     }
 }
diff --git a/src/SumUp/Services/Transactions.php b/src/SumUp/Services/Transactions.php
index 1356f03..15180d0 100644
--- a/src/SumUp/Services/Transactions.php
+++ b/src/SumUp/Services/Transactions.php
@@ -2,6 +2,8 @@
 
 namespace SumUp\Services;
 
+use SumUp\Exceptions\SumUpSDKException;
+use SumUp\HttpClients\Response;
 use SumUp\HttpClients\SumUpHttpClientInterface;
 use SumUp\Authentication\AccessToken;
 use SumUp\Exceptions\SumUpArgumentException;
@@ -44,17 +46,14 @@ public function __construct(SumUpHttpClientInterface $client, AccessToken $acces
     /**
      * Get single transaction by transaction ID.
      *
-     * @param $transactionId
+     * @param string $transactionId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function findById($transactionId)
+    public function findById(string $transactionId): Response
     {
         if (empty($transactionId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('transaction id'));
@@ -67,17 +66,14 @@ public function findById($transactionId)
     /**
      * Get single transaction by internal ID.
      *
-     * @param $internalId
+     * @param string $internalId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function findByInternalId($internalId)
+    public function findByInternalId(string $internalId): Response
     {
         if (empty($internalId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('internal id'));
@@ -90,17 +86,14 @@ public function findByInternalId($internalId)
     /**
      * Get single transaction by foreign transaction id.
      *
-     * @param $foreignId
+     * @param string $foreignId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function findByForeignId($foreignId)
+    public function findByForeignId(string $foreignId): Response
     {
         if (empty($foreignId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('foreign transaction id'));
@@ -113,17 +106,14 @@ public function findByForeignId($foreignId)
     /**
      * Get single transaction by transaction code.
      *
-     * @param $transactionCode
+     * @param string $transactionCode
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function findByTransactionCode($transactionCode)
+    public function findByTransactionCode(string $transactionCode): Response
     {
         if (empty($transactionCode)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('transaction code'));
@@ -138,14 +128,9 @@ public function findByTransactionCode($transactionCode)
      *
      * @param array $filters
      *
-     * @return \SumUp\HttpClients\Response
-     *
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @return Response
      */
-    public function getTransactionHistory($filters = [])
+    public function getTransactionHistory(array $filters = []): Response
     {
         $filters = array_merge([
             'order' => 'ascending',
@@ -176,18 +161,15 @@ public function getTransactionHistory($filters = [])
     /**
      * Refund a transaction partially or fully.
      *
-     * @param $transactionId
-     * @param null $amount
+     * @param string $transactionId
+     * @param string|null $amount
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function refund($transactionId, $amount = null)
+    public function refund(string $transactionId, ?string $amount = null): Response
     {
         if (empty($transactionId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('transaction id'));
@@ -206,18 +188,15 @@ public function refund($transactionId, $amount = null)
     /**
      * Get a receipt for a transaction.
      *
-     * @param $transactionId
-     * @param $merchantId
+     * @param string $transactionId
+     * @param string $merchantId
      *
-     * @return \SumUp\HttpClients\Response
+     * @return Response
      *
      * @throws SumUpArgumentException
-     * @throws \SumUp\Exceptions\SumUpConnectionException
-     * @throws \SumUp\Exceptions\SumUpResponseException
-     * @throws \SumUp\Exceptions\SumUpAuthenticationException
-     * @throws \SumUp\Exceptions\SumUpSDKException
+     * @throws SumUpSDKException
      */
-    public function getReceipt($transactionId, $merchantId)
+    public function getReceipt(string $transactionId, string $merchantId): Response
     {
         if (empty($transactionId)) {
             throw new SumUpArgumentException(ExceptionMessages::getMissingParamMsg('transaction id'));
diff --git a/src/SumUp/SumUp.php b/src/SumUp/SumUp.php
index 592c0f2..e18d787 100644
--- a/src/SumUp/SumUp.php
+++ b/src/SumUp/SumUp.php
@@ -49,7 +49,7 @@ class SumUp
      *
      * @throws SumUpSDKException
      */
-    public function __construct(array $config = [], SumUpHttpClientInterface $customHttpClient = null)
+    public function __construct(array $config = [], ?SumUpHttpClientInterface $customHttpClient = null)
     {
         $this->appConfig = new ApplicationConfiguration($config);
         $this->client = HttpClientsFactory::createHttpClient($this->appConfig, $customHttpClient);
@@ -70,17 +70,17 @@ public function getAccessToken()
     /**
      * Refresh the access token.
      *
-     * @param string $refreshToken
+     * @param string|null $refreshToken
      *
      * @return Authentication\AccessToken
      *
      * @throws SumUpSDKException
      */
-    public function refreshToken($refreshToken = null)
+    public function refreshToken(?string $refreshToken = null): AccessToken
     {
         if (isset($refreshToken)) {
             $rToken = $refreshToken;
-        } elseif (!isset($refreshToken) && !isset($this->accessToken)) {
+        } elseif (!isset($this->accessToken)) {
             throw new SumUpConfigurationException('There is no refresh token');
         } else {
             $rToken = $this->accessToken->getRefreshToken();
@@ -97,7 +97,7 @@ public function refreshToken($refreshToken = null)
      *
      * @return Authorization
      */
-    public function getAuthorizationService(ApplicationConfigurationInterface $config = null)
+    public function getAuthorizationService(?ApplicationConfigurationInterface $config = null): Authorization
     {
         if (empty($config)) {
             $cfg = $this->appConfig;
@@ -114,7 +114,7 @@ public function getAuthorizationService(ApplicationConfigurationInterface $confi
      *
      * @return Checkouts
      */
-    public function getCheckoutService(AccessToken $accessToken = null)
+    public function getCheckoutService(?AccessToken $accessToken = null): Checkouts
     {
         if (!empty($accessToken)) {
             $accToken = $accessToken;
@@ -131,7 +131,7 @@ public function getCheckoutService(AccessToken $accessToken = null)
      *
      * @return Customers
      */
-    public function getCustomerService(AccessToken $accessToken = null)
+    public function getCustomerService(?AccessToken $accessToken = null): Customers
     {
         if (!empty($accessToken)) {
             $accToken = $accessToken;
@@ -148,7 +148,7 @@ public function getCustomerService(AccessToken $accessToken = null)
      *
      * @return Transactions
      */
-    public function getTransactionService(AccessToken $accessToken = null)
+    public function getTransactionService(?AccessToken $accessToken = null): Transactions
     {
         if (!empty($accessToken)) {
             $accToken = $accessToken;
@@ -165,7 +165,7 @@ public function getTransactionService(AccessToken $accessToken = null)
      *
      * @return Merchant
      */
-    public function getMerchantService(AccessToken $accessToken = null)
+    public function getMerchantService(?AccessToken $accessToken = null): Merchant
     {
         if (!empty($accessToken)) {
             $accToken = $accessToken;
@@ -182,7 +182,7 @@ public function getMerchantService(AccessToken $accessToken = null)
      *
      * @return Payouts
      */
-    public function getPayoutService(AccessToken $accessToken = null)
+    public function getPayoutService(?AccessToken $accessToken = null): Payouts
     {
         if (!empty($accessToken)) {
             $accToken = $accessToken;
@@ -197,7 +197,7 @@ public function getPayoutService(AccessToken $accessToken = null)
      *
      * @return Custom
      */
-    public function getCustomService(AccessToken $accessToken = null)
+    public function getCustomService(?AccessToken $accessToken = null): Custom
     {
         if (!empty($accessToken)) {
             $accToken = $accessToken;
diff --git a/src/SumUp/Utils/ExceptionMessages.php b/src/SumUp/Utils/ExceptionMessages.php
index ac7e951..749d025 100644
--- a/src/SumUp/Utils/ExceptionMessages.php
+++ b/src/SumUp/Utils/ExceptionMessages.php
@@ -12,11 +12,11 @@ class ExceptionMessages
     /**
      * Get formatted message for missing parameter.
      *
-     * @param $missingParamName
+     * @param string $missingParamName
      *
      * @return string
      */
-    public static function getMissingParamMsg($missingParamName)
+    public static function getMissingParamMsg(string $missingParamName): string
     {
         return 'Missing parameter: "' . $missingParamName . '".';
     }
diff --git a/src/SumUp/Utils/Headers.php b/src/SumUp/Utils/Headers.php
index e687a1a..f22baa3 100644
--- a/src/SumUp/Utils/Headers.php
+++ b/src/SumUp/Utils/Headers.php
@@ -17,7 +17,7 @@ class Headers
      *
      * @return array
      */
-    public static function getCTJson()
+    public static function getCTJson(): array
     {
         return ['Content-Type' => 'application/json'];
     }
@@ -27,7 +27,7 @@ public static function getCTJson()
      *
      * @return array
      */
-    public static function getCTForm()
+    public static function getCTForm(): array
     {
         return ['Content-Type' => 'application/x-www-form-urlencoded'];
     }
@@ -39,7 +39,7 @@ public static function getCTForm()
      *
      * @return array
      */
-    public static function getAuth(AccessToken $accessToken)
+    public static function getAuth(AccessToken $accessToken): array
     {
         return ['Authorization' => 'Bearer ' . $accessToken->getValue()];
     }
@@ -49,7 +49,7 @@ public static function getAuth(AccessToken $accessToken)
      *
      * @return array
      */
-    public static function getStandardHeaders()
+    public static function getStandardHeaders(): array
     {
         $headers = self::getCTJson();
         $headers['User-Agent'] = SdkInfo::getUserAgent();