diff --git a/src/Symfony/Component/Workflow/CHANGELOG.md b/src/Symfony/Component/Workflow/CHANGELOG.md
index 891a66c713402..619a4227acdfb 100644
--- a/src/Symfony/Component/Workflow/CHANGELOG.md
+++ b/src/Symfony/Component/Workflow/CHANGELOG.md
@@ -4,6 +4,7 @@ CHANGELOG
 5.1.0
 -----
 
+ * Added parameter type hinting where possible.
  * Added context to `TransitionException` and its child classes whenever they are thrown in `Workflow::apply()`
  * Added `Registry::has()` to check if a workflow exists
  * Added support for `$context[Workflow::DISABLE_ANNOUNCE_EVENT] = true` when calling `workflow->apply()` to not fire the announce event
diff --git a/src/Symfony/Component/Workflow/Definition.php b/src/Symfony/Component/Workflow/Definition.php
index 210e7569579ba..ae8f0b040c3ce 100644
--- a/src/Symfony/Component/Workflow/Definition.php
+++ b/src/Symfony/Component/Workflow/Definition.php
@@ -76,7 +76,7 @@ public function getMetadataStore(): MetadataStoreInterface
         return $this->metadataStore;
     }
 
-    private function setInitialPlaces($places = null)
+    private function setInitialPlaces($places = null): void
     {
         if (!$places) {
             return;
@@ -93,7 +93,7 @@ private function setInitialPlaces($places = null)
         $this->initialPlaces = $places;
     }
 
-    private function addPlace(string $place)
+    private function addPlace(string $place): void
     {
         if (!\count($this->places)) {
             $this->initialPlaces = [$place];
@@ -102,7 +102,7 @@ private function addPlace(string $place)
         $this->places[$place] = $place;
     }
 
-    private function addTransition(Transition $transition)
+    private function addTransition(Transition $transition): void
     {
         $name = $transition->getName();
 
diff --git a/src/Symfony/Component/Workflow/DefinitionBuilder.php b/src/Symfony/Component/Workflow/DefinitionBuilder.php
index 19e9067edda98..eada597f8d75f 100644
--- a/src/Symfony/Component/Workflow/DefinitionBuilder.php
+++ b/src/Symfony/Component/Workflow/DefinitionBuilder.php
@@ -37,10 +37,7 @@ public function __construct(array $places = [], array $transitions = [])
         $this->addTransitions($transitions);
     }
 
-    /**
-     * @return Definition
-     */
-    public function build()
+    public function build(): Definition
     {
         return new Definition($this->places, $this->transitions, $this->initialPlaces, $this->metadataStore);
     }
@@ -50,7 +47,7 @@ public function build()
      *
      * @return $this
      */
-    public function clear()
+    public function clear(): self
     {
         $this->places = [];
         $this->transitions = [];
@@ -65,7 +62,7 @@ public function clear()
      *
      * @return $this
      */
-    public function setInitialPlaces($initialPlaces)
+    public function setInitialPlaces($initialPlaces): self
     {
         $this->initialPlaces = $initialPlaces;
 
@@ -75,7 +72,7 @@ public function setInitialPlaces($initialPlaces)
     /**
      * @return $this
      */
-    public function addPlace(string $place)
+    public function addPlace(string $place): self
     {
         if (!$this->places) {
             $this->initialPlaces = $place;
@@ -91,7 +88,7 @@ public function addPlace(string $place)
      *
      * @return $this
      */
-    public function addPlaces(array $places)
+    public function addPlaces(array $places): self
     {
         foreach ($places as $place) {
             $this->addPlace($place);
@@ -105,7 +102,7 @@ public function addPlaces(array $places)
      *
      * @return $this
      */
-    public function addTransitions(array $transitions)
+    public function addTransitions(array $transitions): self
     {
         foreach ($transitions as $transition) {
             $this->addTransition($transition);
@@ -117,7 +114,7 @@ public function addTransitions(array $transitions)
     /**
      * @return $this
      */
-    public function addTransition(Transition $transition)
+    public function addTransition(Transition $transition): self
     {
         $this->transitions[] = $transition;
 
@@ -127,7 +124,7 @@ public function addTransition(Transition $transition)
     /**
      * @return $this
      */
-    public function setMetadataStore(MetadataStoreInterface $metadataStore)
+    public function setMetadataStore(MetadataStoreInterface $metadataStore): self
     {
         $this->metadataStore = $metadataStore;
 
diff --git a/src/Symfony/Component/Workflow/EventListener/AuditTrailListener.php b/src/Symfony/Component/Workflow/EventListener/AuditTrailListener.php
index 95a9590cef4b2..011ac376daffa 100644
--- a/src/Symfony/Component/Workflow/EventListener/AuditTrailListener.php
+++ b/src/Symfony/Component/Workflow/EventListener/AuditTrailListener.php
@@ -27,26 +27,26 @@ public function __construct(LoggerInterface $logger)
         $this->logger = $logger;
     }
 
-    public function onLeave(Event $event)
+    public function onLeave(Event $event): void
     {
         foreach ($event->getTransition()->getFroms() as $place) {
             $this->logger->info(sprintf('Leaving "%s" for subject of class "%s" in workflow "%s".', $place, \get_class($event->getSubject()), $event->getWorkflowName()));
         }
     }
 
-    public function onTransition(Event $event)
+    public function onTransition(Event $event): void
     {
         $this->logger->info(sprintf('Transition "%s" for subject of class "%s" in workflow "%s".', $event->getTransition()->getName(), \get_class($event->getSubject()), $event->getWorkflowName()));
     }
 
-    public function onEnter(Event $event)
+    public function onEnter(Event $event): void
     {
         foreach ($event->getTransition()->getTos() as $place) {
             $this->logger->info(sprintf('Entering "%s" for subject of class "%s" in workflow "%s".', $place, \get_class($event->getSubject()), $event->getWorkflowName()));
         }
     }
 
-    public static function getSubscribedEvents()
+    public static function getSubscribedEvents(): iterable
     {
         return [
             'workflow.leave' => ['onLeave'],
diff --git a/src/Symfony/Component/Workflow/EventListener/GuardExpression.php b/src/Symfony/Component/Workflow/EventListener/GuardExpression.php
index c7ac9cede77f0..b8f355c7ec062 100644
--- a/src/Symfony/Component/Workflow/EventListener/GuardExpression.php
+++ b/src/Symfony/Component/Workflow/EventListener/GuardExpression.php
@@ -24,12 +24,12 @@ public function __construct(Transition $transition, string $expression)
         $this->expression = $expression;
     }
 
-    public function getTransition()
+    public function getTransition(): Transition
     {
         return $this->transition;
     }
 
-    public function getExpression()
+    public function getExpression(): string
     {
         return $this->expression;
     }
diff --git a/src/Symfony/Component/Workflow/EventListener/GuardListener.php b/src/Symfony/Component/Workflow/EventListener/GuardListener.php
index 4badb75afbd31..1531df7e9fb80 100644
--- a/src/Symfony/Component/Workflow/EventListener/GuardListener.php
+++ b/src/Symfony/Component/Workflow/EventListener/GuardListener.php
@@ -44,7 +44,7 @@ public function __construct(array $configuration, ExpressionLanguage $expression
         $this->validator = $validator;
     }
 
-    public function onTransition(GuardEvent $event, string $eventName)
+    public function onTransition(GuardEvent $event, string $eventName): void
     {
         if (!isset($this->configuration[$eventName])) {
             return;
@@ -63,7 +63,7 @@ public function onTransition(GuardEvent $event, string $eventName)
         }
     }
 
-    private function validateGuardExpression(GuardEvent $event, string $expression)
+    private function validateGuardExpression(GuardEvent $event, string $expression): void
     {
         if (!$this->expressionLanguage->evaluate($expression, $this->getVariables($event))) {
             $blocker = TransitionBlocker::createBlockedByExpressionGuardListener($expression);
diff --git a/src/Symfony/Component/Workflow/Marking.php b/src/Symfony/Component/Workflow/Marking.php
index 974a040b0d74f..7c5db459c1193 100644
--- a/src/Symfony/Component/Workflow/Marking.php
+++ b/src/Symfony/Component/Workflow/Marking.php
@@ -30,22 +30,22 @@ public function __construct(array $representation = [])
         }
     }
 
-    public function mark(string $place)
+    public function mark(string $place): void
     {
         $this->places[$place] = 1;
     }
 
-    public function unmark(string $place)
+    public function unmark(string $place): void
     {
         unset($this->places[$place]);
     }
 
-    public function has(string $place)
+    public function has(string $place): bool
     {
         return isset($this->places[$place]);
     }
 
-    public function getPlaces()
+    public function getPlaces(): array
     {
         return $this->places;
     }
diff --git a/src/Symfony/Component/Workflow/Transition.php b/src/Symfony/Component/Workflow/Transition.php
index f5a19b1049e23..683ca8495d0de 100644
--- a/src/Symfony/Component/Workflow/Transition.php
+++ b/src/Symfony/Component/Workflow/Transition.php
@@ -32,17 +32,17 @@ public function __construct(string $name, $froms, $tos)
         $this->tos = (array) $tos;
     }
 
-    public function getName()
+    public function getName(): string
     {
         return $this->name;
     }
 
-    public function getFroms()
+    public function getFroms(): array
     {
         return $this->froms;
     }
 
-    public function getTos()
+    public function getTos(): array
     {
         return $this->tos;
     }
diff --git a/src/Symfony/Component/Workflow/Validator/StateMachineValidator.php b/src/Symfony/Component/Workflow/Validator/StateMachineValidator.php
index 023ca5df5e047..661c8e09d0470 100644
--- a/src/Symfony/Component/Workflow/Validator/StateMachineValidator.php
+++ b/src/Symfony/Component/Workflow/Validator/StateMachineValidator.php
@@ -19,7 +19,7 @@
  */
 class StateMachineValidator implements DefinitionValidatorInterface
 {
-    public function validate(Definition $definition, string $name)
+    public function validate(Definition $definition, string $name): void
     {
         $transitionFromNames = [];
         foreach ($definition->getTransitions() as $transition) {
diff --git a/src/Symfony/Component/Workflow/Validator/WorkflowValidator.php b/src/Symfony/Component/Workflow/Validator/WorkflowValidator.php
index 32cc80ef1efad..cf112ebbce761 100644
--- a/src/Symfony/Component/Workflow/Validator/WorkflowValidator.php
+++ b/src/Symfony/Component/Workflow/Validator/WorkflowValidator.php
@@ -27,7 +27,7 @@ public function __construct(bool $singlePlace = false)
         $this->singlePlace = $singlePlace;
     }
 
-    public function validate(Definition $definition, string $name)
+    public function validate(Definition $definition, string $name): void
     {
         // Make sure all transitions for one place has unique name.
         $places = array_fill_keys($definition->getPlaces(), []);
diff --git a/src/Symfony/Component/Workflow/Workflow.php b/src/Symfony/Component/Workflow/Workflow.php
index f3b290fc394ee..d4b2d2ba1a878 100644
--- a/src/Symfony/Component/Workflow/Workflow.php
+++ b/src/Symfony/Component/Workflow/Workflow.php
@@ -51,7 +51,7 @@ public function __construct(Definition $definition, MarkingStoreInterface $marki
     /**
      * {@inheritdoc}
      */
-    public function getMarking(object $subject)
+    public function getMarking(object $subject): Marking
     {
         $marking = $this->markingStore->getMarking($subject);
 
@@ -93,7 +93,7 @@ public function getMarking(object $subject)
     /**
      * {@inheritdoc}
      */
-    public function can(object $subject, string $transitionName)
+    public function can(object $subject, string $transitionName): bool
     {
         $transitions = $this->definition->getTransitions();
         $marking = $this->getMarking($subject);
@@ -152,7 +152,7 @@ public function buildTransitionBlockerList(object $subject, string $transitionNa
     /**
      * {@inheritdoc}
      */
-    public function apply(object $subject, string $transitionName, array $context = [])
+    public function apply(object $subject, string $transitionName, array $context = []): Marking
     {
         $marking = $this->getMarking($subject);
 
@@ -220,7 +220,7 @@ public function apply(object $subject, string $transitionName, array $context =
     /**
      * {@inheritdoc}
      */
-    public function getEnabledTransitions(object $subject)
+    public function getEnabledTransitions(object $subject): iterable
     {
         $enabledTransitions = [];
         $marking = $this->getMarking($subject);
@@ -238,7 +238,7 @@ public function getEnabledTransitions(object $subject)
     /**
      * {@inheritdoc}
      */
-    public function getName()
+    public function getName(): string
     {
         return $this->name;
     }
@@ -246,7 +246,7 @@ public function getName()
     /**
      * {@inheritdoc}
      */
-    public function getDefinition()
+    public function getDefinition(): Definition
     {
         return $this->definition;
     }
@@ -254,7 +254,7 @@ public function getDefinition()
     /**
      * {@inheritdoc}
      */
-    public function getMarkingStore()
+    public function getMarkingStore(): MarkingStoreInterface
     {
         return $this->markingStore;
     }
diff --git a/src/Symfony/Component/Workflow/WorkflowInterface.php b/src/Symfony/Component/Workflow/WorkflowInterface.php
index 39d9361a5fe72..80fe23b543d00 100644
--- a/src/Symfony/Component/Workflow/WorkflowInterface.php
+++ b/src/Symfony/Component/Workflow/WorkflowInterface.php
@@ -27,14 +27,14 @@ interface WorkflowInterface
      *
      * @throws LogicException
      */
-    public function getMarking(object $subject);
+    public function getMarking(object $subject): Marking;
 
     /**
      * Returns true if the transition is enabled.
      *
      * @return bool true if the transition is enabled
      */
-    public function can(object $subject, string $transitionName);
+    public function can(object $subject, string $transitionName): bool;
 
     /**
      * Builds a TransitionBlockerList to know why a transition is blocked.
@@ -48,29 +48,20 @@ public function buildTransitionBlockerList(object $subject, string $transitionNa
      *
      * @throws LogicException If the transition is not applicable
      */
-    public function apply(object $subject, string $transitionName, array $context = []);
+    public function apply(object $subject, string $transitionName, array $context = []): Marking;
 
     /**
      * Returns all enabled transitions.
      *
      * @return Transition[] All enabled transitions
      */
-    public function getEnabledTransitions(object $subject);
+    public function getEnabledTransitions(object $subject): iterable;
 
-    /**
-     * @return string
-     */
-    public function getName();
+    public function getName(): string;
 
-    /**
-     * @return Definition
-     */
-    public function getDefinition();
+    public function getDefinition(): Definition;
 
-    /**
-     * @return MarkingStoreInterface
-     */
-    public function getMarkingStore();
+    public function getMarkingStore(): MarkingStoreInterface;
 
     public function getMetadataStore(): MetadataStoreInterface;
 }