diff --git a/Neos.Flow/Classes/Session/Session.php b/Neos.Flow/Classes/Session/Session.php
index 4268fcb078..f5148b1d0f 100644
--- a/Neos.Flow/Classes/Session/Session.php
+++ b/Neos.Flow/Classes/Session/Session.php
@@ -198,7 +198,7 @@ public static function createRemoteFromSessionMetaData(SessionMetaData $sessionM
      * @param array $tags
      * @return Session
      */
-    public static function createFromCookieAndSessionInformation(Cookie $sessionCookie, string $storageIdentifier, int $lastActivityTimestamp, array $tags = [])
+    public static function createFromCookieAndSessionInformation(Cookie $sessionCookie, string $storageIdentifier, int $lastActivityTimestamp, array $tags = []): self
     {
         $session = new static();
         $session->sessionMetaData = new SessionMetaData(
@@ -215,9 +215,8 @@ public static function createFromCookieAndSessionInformation(Cookie $sessionCook
      * Injects the Flow settings
      *
      * @param array $settings Settings of the Flow package
-     * @return void
      */
-    public function injectSettings(array $settings)
+    public function injectSettings(array $settings): void
     {
         $this->sessionCookieName = $settings['session']['name'];
         $this->sessionCookieLifetime = (integer)$settings['session']['cookie']['lifetime'];
@@ -233,9 +232,8 @@ public function injectSettings(array $settings)
      * Injects the (system) logger based on PSR-3.
      *
      * @param LoggerInterface $logger
-     * @return void
      */
-    public function injectLogger(LoggerInterface $logger)
+    public function injectLogger(LoggerInterface $logger): void
     {
         $this->logger = $logger;
     }
@@ -251,10 +249,9 @@ public function getSessionCookie(): Cookie
     /**
      * Tells if the session has been started already.
      *
-     * @return boolean
      * @api
      */
-    public function isStarted()
+    public function isStarted(): bool
     {
         return $this->started;
     }
@@ -266,7 +263,7 @@ public function isStarted()
      * @return boolean true if the session is remote, false if this is the current session
      * @api
      */
-    public function isRemote()
+    public function isRemote(): bool
     {
         return $this->remote;
     }
@@ -274,12 +271,11 @@ public function isRemote()
     /**
      * Starts the session, if it has not been already started
      *
-     * @return void
      * @throws \Exception
      * @see CookieEnabledInterface
      * @api
      */
-    public function start()
+    public function start(): void
     {
         if ($this->started === false) {
             $this->sessionMetaData = SessionMetaData::createWithTimestamp($this->now);
@@ -303,7 +299,7 @@ public function start()
      * @return boolean
      * @api
      */
-    public function canBeResumed()
+    public function canBeResumed(): bool
     {
         if ($this->sessionCookie === null || $this->started === true) {
             return false;
@@ -327,7 +323,7 @@ public function canBeResumed()
      * @return null|integer If a session was resumed, the inactivity of this session since the last request is returned
      * @api
      */
-    public function resume()
+    public function resume(): null|int
     {
         if ($this->started === false && $this->canBeResumed()) {
             $this->started = true;
@@ -363,7 +359,7 @@ public function resume()
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function getId()
+    public function getId(): string
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to retrieve the session identifier, but the session has not been started yet.)', 1351171517);
@@ -380,7 +376,7 @@ public function getId()
      * @throws Exception\OperationNotSupportedException
      * @api
      */
-    public function renewId()
+    public function renewId(): string
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to renew the session identifier, but the session has not been started yet.', 1351182429);
@@ -404,7 +400,7 @@ public function renewId()
      * @return mixed The contents associated with the given key
      * @throws Exception\SessionNotStartedException
      */
-    public function getData($key)
+    public function getData(string $key): mixed
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to get session data, but the session has not been started yet.', 1351162255);
@@ -419,7 +415,7 @@ public function getData($key)
      * @return boolean
      * @throws Exception\SessionNotStartedException
      */
-    public function hasKey($key)
+    public function hasKey(string $key): bool
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to check a session data entry, but the session has not been started yet.', 1352488661);
@@ -432,12 +428,11 @@ public function hasKey($key)
      *
      * @param string $key The key under which the data should be stored
      * @param mixed $data The data to be stored
-     * @return void
      * @throws Exception\DataNotSerializableException
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function putData($key, $data)
+    public function putData(string $key, mixed $data): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to create a session data entry, but the session has not been started yet.', 1351162259);
@@ -459,7 +454,7 @@ public function putData($key, $data)
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function getLastActivityTimestamp()
+    public function getLastActivityTimestamp(): int
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to retrieve the last activity timestamp of a session which has not been started yet.', 1354290378);
@@ -474,12 +469,11 @@ public function getLastActivityTimestamp()
      * recommended to use namespaced tags such as "Acme-Demo-MySpecialTag".
      *
      * @param string $tag The tag – must match be a valid cache frontend tag
-     * @return void
      * @throws Exception\SessionNotStartedException
      * @throws \InvalidArgumentException
      * @api
      */
-    public function addTag($tag)
+    public function addTag(string $tag): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to tag a session which has not been started yet.', 1355143533);
@@ -494,11 +488,10 @@ public function addTag($tag)
      * Removes the specified tag from this session.
      *
      * @param string $tag The tag – must match be a valid cache frontend tag
-     * @return void
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function removeTag($tag)
+    public function removeTag(string $tag): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to tag a session which has not been started yet.', 1355150140);
@@ -510,11 +503,11 @@ public function removeTag($tag)
     /**
      * Returns the tags this session has been tagged with.
      *
-     * @return array The tags or an empty array if there aren't any
+     * @return string[] The tags or an empty array if there aren't any
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function getTags()
+    public function getTags(): array
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to retrieve tags from a session which has not been started yet.', 1355141501);
@@ -525,10 +518,9 @@ public function getTags()
     /**
      * Updates the last activity time to "now".
      *
-     * @return void
      * @throws Exception\SessionNotStartedException
      */
-    public function touch()
+    public function touch(): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to touch a session, but the session has not been started yet.', 1354284318);
@@ -545,10 +537,9 @@ public function touch()
     /**
      * Explicitly writes and closes the session
      *
-     * @return void
      * @api
      */
-    public function close()
+    public function close(): void
     {
         $this->shutdownObject();
     }
@@ -557,11 +548,10 @@ public function close()
      * Explicitly destroys all session data
      *
      * @param string $reason A reason for destroying the session – used by the LoggingAspect
-     * @return void
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function destroy($reason = null)
+    public function destroy(string $reason = null): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('Tried to destroy a session which has not been started yet.', 1351162668);
@@ -583,13 +573,12 @@ public function destroy($reason = null)
      * This method must not be called manually – it is invoked by Flow's object
      * management.
      *
-     * @return void
      * @throws Exception\DataNotSerializableException
      * @throws Exception\SessionNotStartedException
      * @throws NotSupportedByBackendException
      * @throws \Neos\Cache\Exception
      */
-    public function shutdownObject()
+    public function shutdownObject(): void
     {
         if ($this->started === true && $this->remote === false) {
             if ($this->sessionMetaDataStore->has($this->sessionMetaData->sessionIdentifier)) {
@@ -605,7 +594,7 @@ public function shutdownObject()
      *
      * @return boolean true if the session expired, false if not
      */
-    protected function autoExpire()
+    protected function autoExpire(): bool
     {
         $lastActivitySecondsAgo = $this->now - $this->sessionMetaData->lastActivityTimestamp;
         $expired = false;
@@ -627,9 +616,8 @@ protected function autoExpire()
      * The session cache entry is also tagged with "session", the session identifier
      * and any custom tags of this session, prefixed with TAG_PREFIX.
      *
-     * @return void
      */
-    protected function writeSessionMetaDataCacheEntry()
+    protected function writeSessionMetaDataCacheEntry(): void
     {
         $this->sessionMetaDataStore->store($this->sessionMetaData);
     }
diff --git a/Neos.Flow/Classes/Session/SessionInterface.php b/Neos.Flow/Classes/Session/SessionInterface.php
index e1cfafd104..1025599869 100644
--- a/Neos.Flow/Classes/Session/SessionInterface.php
+++ b/Neos.Flow/Classes/Session/SessionInterface.php
@@ -21,28 +21,28 @@ interface SessionInterface
      *
      * @return boolean
      */
-    public function isStarted();
+    public function isStarted(): bool;
 
     /**
      * Starts the session, if is has not been already started
      *
      * @return void
      */
-    public function start();
+    public function start(): void;
 
     /**
      * Returns true if there is a session that can be resumed. false otherwise
      *
      * @return boolean
      */
-    public function canBeResumed();
+    public function canBeResumed(): bool;
 
     /**
      * Resumes an existing session, if any.
      *
-     * @return void
+     * @return null|int
      */
-    public function resume();
+    public function resume(): null|int;
 
     /**
      * Returns the current session ID.
@@ -50,7 +50,7 @@ public function resume();
      * @return string The current session ID
      * @throws Exception\SessionNotStartedException
      */
-    public function getId();
+    public function getId(): string;
 
     /**
      * Generates and propagates a new session ID and transfers all existing data
@@ -60,7 +60,7 @@ public function getId();
      *
      * @return string The new session ID
      */
-    public function renewId();
+    public function renewId(): string;
 
     /**
      * Returns the content (mixed) associated with the given key.
@@ -69,7 +69,7 @@ public function renewId();
      * @return mixed The contents associated with the given key
      * @throws Exception\SessionNotStartedException
      */
-    public function getData($key);
+    public function getData(string $key): mixed;
 
     /**
      * Returns true if $key is available.
@@ -77,7 +77,7 @@ public function getData($key);
      * @param string $key
      * @return boolean
      */
-    public function hasKey($key);
+    public function hasKey(string $key): bool;
 
     /**
      * Stores the given data under the given key in the session
@@ -87,7 +87,7 @@ public function hasKey($key);
      * @return void
      * @throws Exception\SessionNotStartedException
      */
-    public function putData($key, $data);
+    public function putData(string $key, mixed $data): void;
 
     /**
      * Tags this session with the given tag.
@@ -101,7 +101,7 @@ public function putData($key, $data);
      * @throws \InvalidArgumentException
      * @api
      */
-    public function addTag($tag);
+    public function addTag(string $tag): void;
 
     /**
      * Removes the specified tag from this session.
@@ -111,16 +111,16 @@ public function addTag($tag);
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function removeTag($tag);
+    public function removeTag(string $tag): void;
 
     /**
      * Returns the tags this session has been tagged with.
      *
-     * @return array The tags or an empty array if there aren't any
+     * @return string[] The tags or an empty array if there aren't any
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function getTags();
+    public function getTags(): array;
 
     /**
      * Updates the last activity time to "now".
@@ -128,7 +128,7 @@ public function getTags();
      * @return void
      * @api
      */
-    public function touch();
+    public function touch(): void;
 
     /**
      * Returns the unix time stamp marking the last point in time this session has
@@ -140,7 +140,7 @@ public function touch();
      * @return integer unix timestamp
      * @api
      */
-    public function getLastActivityTimestamp();
+    public function getLastActivityTimestamp(): int;
 
     /**
      * Explicitly writes (persists) and closes the session
@@ -148,7 +148,7 @@ public function getLastActivityTimestamp();
      * @return void
      * @throws Exception\SessionNotStartedException
      */
-    public function close();
+    public function close(): void;
 
     /**
      * Explicitly destroys all session data
@@ -157,5 +157,5 @@ public function close();
      * @return void
      * @throws Exception\SessionNotStartedException
      */
-    public function destroy($reason = null);
+    public function destroy(string $reason = null): void;
 }
diff --git a/Neos.Flow/Classes/Session/TransientSession.php b/Neos.Flow/Classes/Session/TransientSession.php
index 3e7b81f2a7..2aa51ea8da 100644
--- a/Neos.Flow/Classes/Session/TransientSession.php
+++ b/Neos.Flow/Classes/Session/TransientSession.php
@@ -61,17 +61,15 @@ class TransientSession implements SessionInterface
      *
      * @return boolean
      */
-    public function isStarted()
+    public function isStarted(): bool
     {
         return $this->started;
     }
 
     /**
      * Starts the session, if it has not been already started
-     *
-     * @return void
      */
-    public function start()
+    public function start(): void
     {
         $this->sessionId = Algorithms::generateRandomString(13);
         $this->started = true;
@@ -79,24 +77,21 @@ public function start()
 
     /**
      * Returns true if there is a session that can be resumed. false otherwise
-     *
-     * @return boolean
      */
-    public function canBeResumed()
+    public function canBeResumed(): bool
     {
         return true;
     }
 
     /**
      * Resumes an existing session, if any.
-     *
-     * @return void
      */
-    public function resume()
+    public function resume(): null|int
     {
         if ($this->started === false) {
             $this->start();
         }
+        return null;
     }
 
     /**
@@ -105,7 +100,7 @@ public function resume()
      *
      * @return string The new session ID
      */
-    public function renewId()
+    public function renewId(): string
     {
         $this->sessionId = Algorithms::generateRandomString(13);
         return $this->sessionId;
@@ -117,7 +112,7 @@ public function renewId()
      * @return string The current session ID
      * @throws Exception\SessionNotStartedException
      */
-    public function getId()
+    public function getId(): string
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1218034659);
@@ -132,7 +127,7 @@ public function getId()
      * @return mixed The data associated with the given key or NULL
      * @throws Exception\SessionNotStartedException
      */
-    public function getData($key)
+    public function getData(string $key): mixed
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1218034660);
@@ -144,9 +139,8 @@ public function getData($key)
      * Returns true if $key is available.
      *
      * @param string $key
-     * @return boolean
      */
-    public function hasKey($key)
+    public function hasKey(string $key): bool
     {
         return array_key_exists($key, $this->data);
     }
@@ -155,11 +149,10 @@ public function hasKey($key)
      * Stores the given data under the given key in the session
      *
      * @param string $key The key under which the data should be stored
-     * @param object $data The data to be stored
-     * @return void
+     * @param mixed $data The data to be stored
      * @throws Exception\SessionNotStartedException
      */
-    public function putData($key, $data)
+    public function putData(string $key, mixed $data): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1218034661);
@@ -170,10 +163,9 @@ public function putData($key, $data)
     /**
      * Closes the session
      *
-     * @return void
      * @throws Exception\SessionNotStartedException
      */
-    public function close()
+    public function close(): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1218034662);
@@ -185,10 +177,9 @@ public function close()
      * Explicitly destroys all session data
      *
      * @param string $reason A reason for destroying the session – used by the LoggingAspect
-     * @return void
      * @throws Exception\SessionNotStartedException
      */
-    public function destroy($reason = null)
+    public function destroy(string $reason = null): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1218034663);
@@ -201,18 +192,15 @@ public function destroy($reason = null)
      * No operation for transient session.
      *
      * @param Bootstrap $bootstrap
-     * @return void
      */
-    public static function destroyAll(Bootstrap $bootstrap)
+    public static function destroyAll(Bootstrap $bootstrap): void
     {
     }
 
     /**
      * No operation for transient session.
-     *
-     * @return void
      */
-    public function collectGarbage()
+    public function collectGarbage(): void
     {
     }
 
@@ -222,7 +210,7 @@ public function collectGarbage()
      *
      * @return integer unix timestamp
      */
-    public function getLastActivityTimestamp()
+    public function getLastActivityTimestamp(): int
     {
         if ($this->lastActivityTimestamp === null) {
             $this->touch();
@@ -232,10 +220,8 @@ public function getLastActivityTimestamp()
 
     /**
      * Updates the last activity time to "now".
-     *
-     * @return void
      */
-    public function touch()
+    public function touch(): void
     {
         $this->lastActivityTimestamp = time();
     }
@@ -247,12 +233,11 @@ public function touch()
      * recommended to use namespaced tags such as "Acme-Demo-MySpecialTag".
      *
      * @param string $tag The tag – must match be a valid cache frontend tag
-     * @return void
      * @throws Exception\SessionNotStartedException
      * @throws \InvalidArgumentException
      * @api
      */
-    public function addTag($tag)
+    public function addTag(string $tag): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1422551048);
@@ -264,11 +249,10 @@ public function addTag($tag)
      * Removes the specified tag from this session.
      *
      * @param string $tag The tag – must match be a valid cache frontend tag
-     * @return void
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function removeTag($tag)
+    public function removeTag(string $tag): void
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1422551049);
@@ -281,11 +265,11 @@ public function removeTag($tag)
     /**
      * Returns the tags this session has been tagged with.
      *
-     * @return array The tags or an empty array if there aren't any
+     * @return string[] The tags or an empty array if there aren't any
      * @throws Exception\SessionNotStartedException
      * @api
      */
-    public function getTags()
+    public function getTags(): array
     {
         if ($this->started !== true) {
             throw new Exception\SessionNotStartedException('The session has not been started yet.', 1422551050);