diff --git a/.github/workflows/phpci.yml b/.github/workflows/phpci.yml
index 0bf5e04..ce01315 100644
--- a/.github/workflows/phpci.yml
+++ b/.github/workflows/phpci.yml
@@ -67,6 +67,7 @@ jobs:
                 command:
                     - "test:phpcs"
                     - "test:phpcompatibility"
+                    - "test:phpstan"
                 php-version:
                     - "7.4"
                     - "8.0"
diff --git a/Classes/Action/Ajax/ContentRendererAction.php b/Classes/Action/Ajax/ContentRendererAction.php
index 6993a03..7be9955 100644
--- a/Classes/Action/Ajax/ContentRendererAction.php
+++ b/Classes/Action/Ajax/ContentRendererAction.php
@@ -39,9 +39,6 @@ class ContentRendererAction
 {
     /**
      * Entry point.
-     *
-     * @param array current data record, either a tt_content element or page record
-     * @param string table name, either "pages" or "tt_content"
      */
     public function renderContent()
     {
diff --git a/Classes/Action/FlashMessageAction.php b/Classes/Action/FlashMessageAction.php
index aa21e70..f550636 100644
--- a/Classes/Action/FlashMessageAction.php
+++ b/Classes/Action/FlashMessageAction.php
@@ -40,10 +40,9 @@ class FlashMessageAction extends AbstractAction
     /**
      * @param RequestInterface $request
      *
-     * @return null
+     * @return null|string
      *
      * @throws \Sys25\RnBase\Exception\SkipActionException
-     * @throws \tx_rnbase_exception_Skip
      */
     protected function handleRequest(RequestInterface $request)
     {
diff --git a/Classes/Action/MarkdownAction.php b/Classes/Action/MarkdownAction.php
index 3b2edbf..5bfd3b7 100644
--- a/Classes/Action/MarkdownAction.php
+++ b/Classes/Action/MarkdownAction.php
@@ -93,7 +93,7 @@ protected function auth()
             TYPO3::getBEUserUID()
             // check crypt auth users from ts
             || (
-                $_SERVER['PHP_AUTH_USER'] ?? false
+            ($_SERVER['PHP_AUTH_USER'] ?? false)
                 && isset($auth[$_SERVER['PHP_AUTH_USER']])
                 && $auth[$_SERVER['PHP_AUTH_USER']] === crypt($_SERVER['PHP_AUTH_PW'], $auth[$_SERVER['PHP_AUTH_USER']])
             );
diff --git a/Classes/Action/ShowTemplateAction.php b/Classes/Action/ShowTemplateAction.php
index e58e725..60c17af 100644
--- a/Classes/Action/ShowTemplateAction.php
+++ b/Classes/Action/ShowTemplateAction.php
@@ -43,7 +43,7 @@ class ShowTemplateAction extends AbstractAction
     /**
      * @param RequestInterface $request
      *
-     * @return null
+     * @return null|string
      */
     protected function handleRequest(RequestInterface $request)
     {
@@ -57,7 +57,7 @@ protected function handleRequest(RequestInterface $request)
     /**
      * Returns the data to render for the view.
      *
-     * @return array
+     * @return BaseModel
      */
     protected function getItem()
     {
@@ -72,7 +72,7 @@ protected function getItem()
     /**
      * Returns the data to render for the view.
      *
-     * @return array
+     * @return array|null
      */
     protected function getData()
     {
diff --git a/Classes/ErrorHandler/ErrorHandler.php b/Classes/ErrorHandler/ErrorHandler.php
index d7d36fc..377ed8d 100644
--- a/Classes/ErrorHandler/ErrorHandler.php
+++ b/Classes/ErrorHandler/ErrorHandler.php
@@ -147,6 +147,8 @@ public function handleFatalError()
 
             return true;
         }
+
+        return false;
     }
 
     /**
@@ -164,7 +166,7 @@ protected function getLastError()
      *
      * @param string $exceptionMessage
      *
-     * @return \Sys25\RnBase\Typo3Wrapper\Core\Error\Exception
+     * @return \Exception
      */
     protected function getTypo3Exception($exceptionMessage)
     {
diff --git a/Classes/ErrorHandler/ExceptionHandler.php b/Classes/ErrorHandler/ExceptionHandler.php
index dbb4075..d1e1c11 100644
--- a/Classes/ErrorHandler/ExceptionHandler.php
+++ b/Classes/ErrorHandler/ExceptionHandler.php
@@ -284,7 +284,7 @@ protected function echoExceptionPageAndExit($absoluteExceptionPageUrl)
     /**
      * Methode ist in TYPO3 4.5.x noch nicht vorhanden. Daher selbst eingefügt.
      *
-     * @param \Exception $exception
+     * @param \Throwable|\Exception $exception
      */
     protected function sendStatusHeadersEnvironment($exception)
     {
diff --git a/Classes/Routing/Aspect/StaticNumberRangeMapper.php b/Classes/Routing/Aspect/StaticNumberRangeMapper.php
index ad9a04d..c32b4ff 100644
--- a/Classes/Routing/Aspect/StaticNumberRangeMapper.php
+++ b/Classes/Routing/Aspect/StaticNumberRangeMapper.php
@@ -66,12 +66,12 @@ class StaticNumberRangeMapper implements StaticMappableAspectInterface
     public const MAX_PAGE_LIMIT = 1000;
 
     /**
-     * @var string
+     * @var int
      */
     protected $start;
 
     /**
-     * @var string
+     * @var int
      */
     protected $end;
 
diff --git a/Classes/Session/FlashMessageStorage.php b/Classes/Session/FlashMessageStorage.php
index 95d7017..a183054 100644
--- a/Classes/Session/FlashMessageStorage.php
+++ b/Classes/Session/FlashMessageStorage.php
@@ -193,7 +193,7 @@ public function getMessages()
      * @param string $method
      * @param array  $args
      *
-     * @return self
+     * @return self|null
      *
      * @throws RuntimeException If level or method does not exists
      */
@@ -209,5 +209,7 @@ public static function __callStatic($method, $args)
         if (defined('self::'.$const)) {
             return self::getInstance()->addMessage($args[0], constant('self::'.$const));
         }
+
+        return null;
     }
 }
diff --git a/Classes/Updates/MigrateOldMktoolsPlugins.php b/Classes/Updates/MigrateOldMktoolsPlugins.php
index 6e6f5aa..4ce5eb6 100644
--- a/Classes/Updates/MigrateOldMktoolsPlugins.php
+++ b/Classes/Updates/MigrateOldMktoolsPlugins.php
@@ -132,7 +132,7 @@ protected function getPreparedQueryBuilder(): QueryBuilder
     {
         $queryBuilder = $this->getConnectionPool()->getQueryBuilderForTable(self::TABLE_NAME);
         $queryBuilder->getRestrictions()->removeAll();
-        $whereConditions = [];
+        $whereConditionsForOldClasses = [];
         foreach ($this->classMappings as $oldClass => $newClass) {
             $whereConditionsForOldClasses[] = $queryBuilder->expr()->like(
                 'pi_flexform',
diff --git a/Classes/Utility/CacheUtility.php b/Classes/Utility/CacheUtility.php
index 368f089..b5ff710 100644
--- a/Classes/Utility/CacheUtility.php
+++ b/Classes/Utility/CacheUtility.php
@@ -73,7 +73,7 @@ public static function isApcUsed()
     }
 
     /**
-     * @return bool
+     * @return string
      */
     public static function getApcCacheBackendClass()
     {
diff --git a/Classes/Utility/Menu/Processor/TranslatedRecords.php b/Classes/Utility/Menu/Processor/TranslatedRecords.php
index 5ed8302..bbf625d 100644
--- a/Classes/Utility/Menu/Processor/TranslatedRecords.php
+++ b/Classes/Utility/Menu/Processor/TranslatedRecords.php
@@ -122,7 +122,7 @@ public function process(array $menuItems, array $typoScriptConfiguration): array
      *
      * @param string $value
      *
-     * @return bool
+     * @return bool|string
      */
     public function processEmptyIfRecordNotExists($value, array $typoScriptConfiguration)
     {
@@ -195,7 +195,7 @@ protected function handleDisablingOfMenuItemForNotTranslatedRecord(
         $typoScriptFrontendController = $this->getTypoScriptFrontendController();
 
         $pageTranslationVisibility = new PageTranslationVisibility(
-            (int) $typoScriptFrontendController->page['l18n_cfg'] ?? 0
+            (int) ($typoScriptFrontendController->page['l18n_cfg'] ?? 0)
         );
 
         if ($pageTranslationVisibility->shouldHideTranslationIfNoTranslatedRecordExists()
diff --git a/Classes/Utility/Misc.php b/Classes/Utility/Misc.php
index 337f518..c8a2ba1 100644
--- a/Classes/Utility/Misc.php
+++ b/Classes/Utility/Misc.php
@@ -79,7 +79,7 @@ public static function isAjaxContentRendererActive()
     }
 
     /**
-     * @return string
+     * @return string|int
      */
     public static function getExceptionPage()
     {
@@ -103,7 +103,7 @@ public static function getConfigurations($staticPath, $additionalPath = '')
             );
         }
 
-        $pageTSconfig = BackendUtility::getPagesTSconfig(0, 1);
+        $pageTSconfig = \TYPO3\CMS\Backend\Utility\BackendUtility::getPagesTSconfig(0);
         $config = Arrays::mergeRecursiveWithOverrule(
             (array) $pageTSconfig['config.']['tx_mktools.'],
             (array) $pageTSconfig['plugin.']['tx_mktools.']
@@ -118,16 +118,6 @@ public static function getConfigurations($staticPath, $additionalPath = '')
         return $configurations;
     }
 
-    /**
-     * @param string $filename
-     *
-     * @return string
-     */
-    private static function getAbsoluteFileName($filename)
-    {
-        return Files::getFileAbsFileName($filename);
-    }
-
     public static function areUnmappedPageTypesAllowed(): bool
     {
         return (bool) self::getExtensionCfgValue('allowUnmappedPageTypes');
diff --git a/Classes/Utility/SessionUtility.php b/Classes/Utility/SessionUtility.php
index 5962c14..ec0e29e 100644
--- a/Classes/Utility/SessionUtility.php
+++ b/Classes/Utility/SessionUtility.php
@@ -119,7 +119,7 @@ public static function areCookiesActivated()
         } else {
             // @TODO diesen Abschnitt testen, aber wie (vor allem auf CLI)?
             // Wir versuchen selbst einen Cookie zu setzen.
-            setcookie('cookiesActivated', 1, time() + 3600);
+            setcookie('cookiesActivated', '1', time() + 3600);
             // Wir setzen einen Parameter für den Reload,
             // um einen Infinite Redirect zu verhindern
             // falls keine Cookies erlaubt sind.
diff --git a/Classes/Utility/T3Loader.php b/Classes/Utility/T3Loader.php
index 944c226..6f4154f 100644
--- a/Classes/Utility/T3Loader.php
+++ b/Classes/Utility/T3Loader.php
@@ -37,7 +37,7 @@
 final class T3Loader
 {
     /**
-     * @var array[TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer]
+     * @var array[\TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer]
      */
     private static $cObj = [];
 
diff --git a/Tests/Classes/ContentObject/UserContentObjectTest.php b/Tests/Classes/ContentObject/UserContentObjectTest.php
index 987572d..58aa0c7 100644
--- a/Tests/Classes/ContentObject/UserContentObjectTest.php
+++ b/Tests/Classes/ContentObject/UserContentObjectTest.php
@@ -88,8 +88,6 @@ public function testRenderIfContentShouldNotBeLoadedWithAjax($loadWithAjax, $mkt
 
     /**
      * @param \TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer $contentObject
-     *
-     * @return \PHPUnit_Framework_MockObject_MockObject|\DMK\Mktools\ContentObject\UserContentObject
      */
     protected function initializeFixtures($contentObject)
     {
diff --git a/Tests/Classes/ContentObject/UserInternalContentObjectTest.php b/Tests/Classes/ContentObject/UserInternalContentObjectTest.php
index b07edb2..1a9be98 100644
--- a/Tests/Classes/ContentObject/UserInternalContentObjectTest.php
+++ b/Tests/Classes/ContentObject/UserInternalContentObjectTest.php
@@ -80,8 +80,6 @@ public function testRenderIfContentShouldNotBeLoadedWithAjax($loadWithAjax, $mkt
 
     /**
      * @param \TYPO3\CMS\Frontend\ContentObject\ContentObjectRenderer $contentObject
-     *
-     * @return \PHPUnit_Framework_MockObject_MockObject|\DMK\Mktools\ContentObject\UserInternalContentObject
      */
     protected function initializeFixtures($contentObject)
     {
diff --git a/composer.json b/composer.json
index cc7d6f4..dd02523 100644
--- a/composer.json
+++ b/composer.json
@@ -47,7 +47,10 @@
         "typo3/testing-framework": "^6.0 || ^7.0",
         "phpunit/phpunit": "^9.0",
         "phpcompatibility/php-compatibility": "^9.3.5",
-        "phpspec/prophecy-phpunit": "^2.0"
+        "phpspec/prophecy-phpunit": "^2.0",
+        "phpstan/phpstan": "^1.1",
+        "saschaegerer/phpstan-typo3": "^1.9",
+        "phpstan/extension-installer": "*"
     },
     "autoload": {
         "psr-4": {
@@ -109,6 +112,10 @@
             "[ -e .Build/bin/phpcs ] || composer update",
             ".Build/bin/phpcs --ignore=.Build/*,Resources/Public/* -p . --standard=.Build/vendor/phpcompatibility/php-compatibility/PHPCompatibility --runtime-set testVersion 8.2"
         ],
+        "test:phpstan": [
+            "[ -e .Build/bin/phpstan ] || composer update --ansi",
+            ".Build/bin/phpstan analyse -c phpstan.neon --memory-limit=512M --ansi"
+        ],
         "test:phpunit": [
             "[ -e .Build/bin/phpunit ] || composer update --ansi",
             "export TYPO3_PATH_APP=$PWD/.Build && export TYPO3_PATH_WEB=$PWD/.Build/Web && .Build/bin/phpunit -c phpunit.xml.dist --colors=always"
@@ -116,6 +123,7 @@
         "test": [
             "@test:phpcs",
             "@test:phpcompatibility",
+            "@test:phpstan",
             "@test:phpunit"
         ],
         "fix:phpcs": [
diff --git a/phpstan.neon b/phpstan.neon
new file mode 100644
index 0000000..c2fd74b
--- /dev/null
+++ b/phpstan.neon
@@ -0,0 +1,34 @@
+parameters:
+    level: 5
+    bootstrapFiles:
+        - "%currentWorkingDirectory%/.Build/vendor/autoload.php"
+        - "%currentWorkingDirectory%/Resources/Private/PHP/Composer/autoload.php"
+    paths:
+        - %currentWorkingDirectory%/Classes
+        - %currentWorkingDirectory%/ext_localconf.php
+        - %currentWorkingDirectory%/ext_emconf.php
+    reportUnmatchedIgnoredErrors: false
+    ignoreErrors:
+            -
+                message: "#^Constant LF not found\\.$#"
+                count: 1
+                path: Classes/Command/MigrateSwitchableControllerActionsCommand.php
+
+            -
+                message: "#^Call to static method addRulesForFrontend\\(\\) on an unknown class DMK\\\\MkSanitizedParameters\\\\Rules\\.$#"
+                count: 1
+                path: ext_localconf.php
+            -
+                message: "#^Call to method checkRecord\\(\\) on an unknown class TYPO3\\\\CMS\\\\Frontend\\\\Page\\\\PageRepository\\.$#"
+                count: 1
+                path: Classes/Action/Ajax/ContentRendererAction.php
+
+            -
+                message: "#^Call to method getLanguageOverlay\\(\\) on an unknown class TYPO3\\\\CMS\\\\Frontend\\\\Page\\\\PageRepository\\.$#"
+                count: 1
+                path: Classes/Utility/Menu/Processor/TranslatedRecords.php
+
+            -
+                message: "#^Call to method getRecordOverlay\\(\\) on an unknown class TYPO3\\\\CMS\\\\Frontend\\\\Page\\\\PageRepository\\.$#"
+                count: 1
+                path: Classes/Utility/Menu/Processor/TranslatedRecords.php