diff --git a/src/Symfony/Component/Workflow/CHANGELOG.md b/src/Symfony/Component/Workflow/CHANGELOG.md
index 1bdca90c9b..79996f4626 100644
--- a/src/Symfony/Component/Workflow/CHANGELOG.md
+++ b/src/Symfony/Component/Workflow/CHANGELOG.md
@@ -4,10 +4,12 @@ CHANGELOG
 4.1.0
 -----
 
- * Deprecate the usage of `add(Workflow $workflow, $supportStrategy)` in `Workflow/Registry`, use `addWorkflow(WorkflowInterface, $supportStrategy)` instead.  
+ * Deprecate the usage of `add(Workflow $workflow, $supportStrategy)` in `Workflow/Registry`, use `addWorkflow(WorkflowInterface, $supportStrategy)` instead.
  * Deprecate the usage of `SupportStrategyInterface`, use `WorkflowSupportStrategyInterface` instead.
  * The `Workflow` class now implements `WorkflowInterface`.
  * Deprecated the class `ClassInstanceSupportStrategy` in favor of the class `InstanceOfSupportStrategy`.
+ * Added TransitionBlockers as a way to pass around reasons why exactly
+   transitions can't be made.
 
 4.0.0
 -----
diff --git a/src/Symfony/Component/Workflow/Event/GuardEvent.php b/src/Symfony/Component/Workflow/Event/GuardEvent.php
index bf4b6f3971..791457bb92 100644
--- a/src/Symfony/Component/Workflow/Event/GuardEvent.php
+++ b/src/Symfony/Component/Workflow/Event/GuardEvent.php
@@ -11,21 +11,52 @@
 
 namespace Symfony\Component\Workflow\Event;
 
+use Symfony\Component\Workflow\Marking;
+use Symfony\Component\Workflow\Transition;
+use Symfony\Component\Workflow\TransitionBlocker;
+use Symfony\Component\Workflow\TransitionBlockerList;
+
 /**
  * @author Fabien Potencier <fabien@symfony.com>
  * @author Grégoire Pineau <lyrixx@lyrixx.info>
  */
 class GuardEvent extends Event
 {
-    private $blocked = false;
+    private $transitionBlockerList;
 
-    public function isBlocked()
+    /**
+     * {@inheritdoc}
+     */
+    public function __construct($subject, Marking $marking, Transition $transition, $workflowName = 'unnamed')
     {
-        return $this->blocked;
+        parent::__construct($subject, $marking, $transition, $workflowName);
+
+        $this->transitionBlockerList = new TransitionBlockerList();
     }
 
-    public function setBlocked($blocked)
+    public function isBlocked(): bool
     {
-        $this->blocked = (bool) $blocked;
+        return !$this->transitionBlockerList->isEmpty();
+    }
+
+    public function setBlocked(bool $blocked): void
+    {
+        if (!$blocked) {
+            $this->transitionBlockerList->reset();
+
+            return;
+        }
+
+        $this->transitionBlockerList->add(TransitionBlocker::createUnknown());
+    }
+
+    public function getTransitionBlockerList(): TransitionBlockerList
+    {
+        return $this->transitionBlockerList;
+    }
+
+    public function addTransitionBlocker(TransitionBlocker $transitionBlocker): void
+    {
+        $this->transitionBlockerList->add($transitionBlocker);
     }
 }
diff --git a/src/Symfony/Component/Workflow/EventListener/GuardListener.php b/src/Symfony/Component/Workflow/EventListener/GuardListener.php
index a4114e68e1..912dc5dada 100644
--- a/src/Symfony/Component/Workflow/EventListener/GuardListener.php
+++ b/src/Symfony/Component/Workflow/EventListener/GuardListener.php
@@ -18,6 +18,7 @@ use Symfony\Component\Security\Core\Role\RoleHierarchyInterface;
 use Symfony\Component\Validator\Validator\ValidatorInterface;
 use Symfony\Component\Workflow\Event\GuardEvent;
 use Symfony\Component\Workflow\Exception\InvalidTokenConfigurationException;
+use Symfony\Component\Workflow\TransitionBlocker;
 
 /**
  * @author Grégoire Pineau <lyrixx@lyrixx.info>
@@ -49,8 +50,11 @@ class GuardListener
             return;
         }
 
-        if (!$this->expressionLanguage->evaluate($this->configuration[$eventName], $this->getVariables($event))) {
-            $event->setBlocked(true);
+        $expression = $this->configuration[$eventName];
+
+        if (!$this->expressionLanguage->evaluate($expression, $this->getVariables($event))) {
+            $blocker = TransitionBlocker::createBlockedByExpressionGuardListener($expression);
+            $event->addTransitionBlocker($blocker);
         }
     }
 
diff --git a/src/Symfony/Component/Workflow/Exception/NotEnabledTransitionException.php b/src/Symfony/Component/Workflow/Exception/NotEnabledTransitionException.php
new file mode 100644
index 0000000000..48a8c260ff
--- /dev/null
+++ b/src/Symfony/Component/Workflow/Exception/NotEnabledTransitionException.php
@@ -0,0 +1,36 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Workflow\Exception;
+
+use Symfony\Component\Workflow\TransitionBlockerList;
+
+/**
+ * Thrown by Workflow when a not enabled transition is applied on a subject.
+ *
+ * @author Grégoire Pineau <lyrixx@lyrixx.info>
+ */
+class NotEnabledTransitionException extends LogicException
+{
+    private $transitionBlockerList;
+
+    public function __construct(string $transitionName, string $workflowName, TransitionBlockerList $transitionBlockerList)
+    {
+        parent::__construct(sprintf('Transition "%s" is not enabled for workflow "%s".', $transitionName, $workflowName));
+
+        $this->transitionBlockerList = $transitionBlockerList;
+    }
+
+    public function getTransitionBlockerList(): TransitionBlockerList
+    {
+        return $this->transitionBlockerList;
+    }
+}
diff --git a/src/Symfony/Component/Workflow/Exception/UndefinedTransitionException.php b/src/Symfony/Component/Workflow/Exception/UndefinedTransitionException.php
new file mode 100644
index 0000000000..04022e90cc
--- /dev/null
+++ b/src/Symfony/Component/Workflow/Exception/UndefinedTransitionException.php
@@ -0,0 +1,25 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Workflow\Exception;
+
+/**
+ * Thrown by Workflow when an undefined transition is applied on a subject.
+ *
+ * @author Grégoire Pineau <lyrixx@lyrixx.info>
+ */
+class UndefinedTransitionException extends LogicException
+{
+    public function __construct(string $transitionName, string $workflowName)
+    {
+        parent::__construct(sprintf('Transition "%s" is not defined for workflow "%s".', $transitionName, $workflowName));
+    }
+}
diff --git a/src/Symfony/Component/Workflow/Tests/WorkflowTest.php b/src/Symfony/Component/Workflow/Tests/WorkflowTest.php
index 547fb03e82..4451357d53 100644
--- a/src/Symfony/Component/Workflow/Tests/WorkflowTest.php
+++ b/src/Symfony/Component/Workflow/Tests/WorkflowTest.php
@@ -7,10 +7,12 @@ use Symfony\Component\EventDispatcher\EventDispatcher;
 use Symfony\Component\Workflow\Definition;
 use Symfony\Component\Workflow\Event\Event;
 use Symfony\Component\Workflow\Event\GuardEvent;
+use Symfony\Component\Workflow\Exception\NotEnabledTransitionException;
 use Symfony\Component\Workflow\Marking;
 use Symfony\Component\Workflow\MarkingStore\MarkingStoreInterface;
 use Symfony\Component\Workflow\MarkingStore\MultipleStateMarkingStore;
 use Symfony\Component\Workflow\Transition;
+use Symfony\Component\Workflow\TransitionBlocker;
 use Symfony\Component\Workflow\Workflow;
 
 class WorkflowTest extends TestCase
@@ -162,20 +164,6 @@ class WorkflowTest extends TestCase
         $this->assertSame(array('workflow_name.guard.t3'), $dispatchedEvents);
     }
 
-    /**
-     * @expectedException \Symfony\Component\Workflow\Exception\LogicException
-     * @expectedExceptionMessage Unable to apply transition "t2" for workflow "unnamed".
-     */
-    public function testApplyWithImpossibleTransition()
-    {
-        $definition = $this->createComplexWorkflowDefinition();
-        $subject = new \stdClass();
-        $subject->marking = null;
-        $workflow = new Workflow($definition, new MultipleStateMarkingStore());
-
-        $workflow->apply($subject, 't2');
-    }
-
     public function testCanWithSameNameTransition()
     {
         $definition = $this->createWorkflowWithSameNameTransition();
@@ -193,6 +181,99 @@ class WorkflowTest extends TestCase
         $this->assertTrue($workflow->can($subject, 'to_a'));
     }
 
+    /**
+     * @expectedException \Symfony\Component\Workflow\Exception\UndefinedTransitionException
+     * @expectedExceptionMessage Transition "404 Not Found" is not defined for workflow "unnamed".
+     */
+    public function testBuildTransitionBlockerListReturnsUndefinedTransition()
+    {
+        $definition = $this->createSimpleWorkflowDefinition();
+        $subject = new \stdClass();
+        $subject->marking = null;
+        $workflow = new Workflow($definition);
+
+        $workflow->buildTransitionBlockerList($subject, '404 Not Found');
+    }
+
+    public function testBuildTransitionBlockerListReturnsReasonsProvidedByMarking()
+    {
+        $definition = $this->createComplexWorkflowDefinition();
+        $subject = new \stdClass();
+        $subject->marking = null;
+        $workflow = new Workflow($definition, new MultipleStateMarkingStore());
+
+        $transitionBlockerList = $workflow->buildTransitionBlockerList($subject, 't2');
+        $this->assertCount(1, $transitionBlockerList);
+        $blockers = iterator_to_array($transitionBlockerList);
+        $this->assertSame('The marking does not enable the transition.', $blockers[0]->getMessage());
+        $this->assertSame('19beefc8-6b1e-4716-9d07-a39bd6d16e34', $blockers[0]->getCode());
+    }
+
+    public function testBuildTransitionBlockerListReturnsReasonsProvidedInGuards()
+    {
+        $definition = $this->createSimpleWorkflowDefinition();
+        $subject = new \stdClass();
+        $subject->marking = null;
+        $dispatcher = new EventDispatcher();
+        $workflow = new Workflow($definition, new MultipleStateMarkingStore(), $dispatcher);
+
+        $dispatcher->addListener('workflow.guard', function (GuardEvent $event) {
+            $event->addTransitionBlocker(new TransitionBlocker('Transition blocker 1', 'blocker_1'));
+            $event->addTransitionBlocker(new TransitionBlocker('Transition blocker 2', 'blocker_2'));
+        });
+        $dispatcher->addListener('workflow.guard', function (GuardEvent $event) {
+            $event->addTransitionBlocker(new TransitionBlocker('Transition blocker 3', 'blocker_3'));
+        });
+        $dispatcher->addListener('workflow.guard', function (GuardEvent $event) {
+            $event->setBlocked(true);
+        });
+
+        $transitionBlockerList = $workflow->buildTransitionBlockerList($subject, 't1');
+        $this->assertCount(4, $transitionBlockerList);
+        $blockers = iterator_to_array($transitionBlockerList);
+        $this->assertSame('Transition blocker 1', $blockers[0]->getMessage());
+        $this->assertSame('blocker_1', $blockers[0]->getCode());
+        $this->assertSame('Transition blocker 2', $blockers[1]->getMessage());
+        $this->assertSame('blocker_2', $blockers[1]->getCode());
+        $this->assertSame('Transition blocker 3', $blockers[2]->getMessage());
+        $this->assertSame('blocker_3', $blockers[2]->getCode());
+        $this->assertSame('Unknown reason.', $blockers[3]->getMessage());
+        $this->assertSame('e8b5bbb9-5913-4b98-bfa6-65dbd228a82a', $blockers[3]->getCode());
+    }
+
+    /**
+     * @expectedException \Symfony\Component\Workflow\Exception\UndefinedTransitionException
+     * @expectedExceptionMessage Transition "404 Not Found" is not defined for workflow "unnamed".
+     */
+    public function testApplyWithNotExisingTransition()
+    {
+        $definition = $this->createComplexWorkflowDefinition();
+        $subject = new \stdClass();
+        $subject->marking = null;
+        $workflow = new Workflow($definition, new MultipleStateMarkingStore());
+
+        $workflow->apply($subject, '404 Not Found');
+    }
+
+    public function testApplyWithNotEnabledTransition()
+    {
+        $definition = $this->createComplexWorkflowDefinition();
+        $subject = new \stdClass();
+        $subject->marking = null;
+        $workflow = new Workflow($definition, new MultipleStateMarkingStore());
+
+        try {
+            $workflow->apply($subject, 't2');
+
+            $this->fail('Should throw an exception');
+        } catch (NotEnabledTransitionException $e) {
+            $this->assertSame('Transition "t2" is not enabled for workflow "unnamed".', $e->getMessage());
+            $this->assertCount(1, $e->getTransitionBlockerList());
+            $list = iterator_to_array($e->getTransitionBlockerList());
+            $this->assertSame('The marking does not enable the transition.', $list[0]->getMessage());
+        }
+    }
+
     public function testApply()
     {
         $definition = $this->createComplexWorkflowDefinition();
diff --git a/src/Symfony/Component/Workflow/TransitionBlocker.php b/src/Symfony/Component/Workflow/TransitionBlocker.php
new file mode 100644
index 0000000000..d5307038a6
--- /dev/null
+++ b/src/Symfony/Component/Workflow/TransitionBlocker.php
@@ -0,0 +1,91 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Workflow;
+
+/**
+ * A reason why a transition cannot be performed for a subject.
+ */
+final class TransitionBlocker
+{
+    const BLOCKED_BY_MARKING = '19beefc8-6b1e-4716-9d07-a39bd6d16e34';
+    const BLOCKED_BY_EXPRESSION_GUARD_LISTENER = '326a1e9c-0c12-11e8-ba89-0ed5f89f718b';
+    const UNKNOWN = 'e8b5bbb9-5913-4b98-bfa6-65dbd228a82a';
+
+    private $message;
+    private $code;
+    private $parameters;
+
+    /**
+     * @param string $code       Code is a machine-readable string, usually an UUID
+     * @param array  $parameters This is useful if you would like to pass around the condition values, that
+     *                           blocked the transition. E.g. for a condition "distance must be larger than
+     *                           5 miles", you might want to pass around the value of 5.
+     */
+    public function __construct(string $message, string $code, array $parameters = array())
+    {
+        $this->message = $message;
+        $this->code = $code;
+        $this->parameters = $parameters;
+    }
+
+    /**
+     * Create a blocker that says the transition cannot be made because it is
+     * not enabled.
+     *
+     * It means the subject is in wrong place (i.e. status):
+     * * If the workflow is a state machine: the subject is not in the previous place of the transition.
+     * * If the workflow is a workflow: the subject is not in all previous places of the transition.
+     */
+    public static function createBlockedByMarking(Marking $marking): self
+    {
+        return new static('The marking does not enable the transition.', self::BLOCKED_BY_MARKING, array(
+            'marking' => $marking,
+        ));
+    }
+
+    /**
+     * Creates a blocker that says the transition cannot be made because it has
+     * been blocked by the expression guard listener.
+     */
+    public static function createBlockedByExpressionGuardListener(string $expression): self
+    {
+        return new static('The expression blocks the transition.', self::BLOCKED_BY_EXPRESSION_GUARD_LISTENER, array(
+            'expression' => $expression,
+        ));
+    }
+
+    /**
+     * Creates a blocker that says the transition cannot be made because of an
+     * unknown reason.
+     *
+     * This blocker code is chiefly for preserving backwards compatibility.
+     */
+    public static function createUnknown(): self
+    {
+        return new static('Unknown reason.', self::UNKNOWN);
+    }
+
+    public function getMessage(): string
+    {
+        return $this->message;
+    }
+
+    public function getCode(): string
+    {
+        return $this->code;
+    }
+
+    public function getParameters(): array
+    {
+        return $this->parameters;
+    }
+}
diff --git a/src/Symfony/Component/Workflow/TransitionBlockerList.php b/src/Symfony/Component/Workflow/TransitionBlockerList.php
new file mode 100644
index 0000000000..85a9888fc3
--- /dev/null
+++ b/src/Symfony/Component/Workflow/TransitionBlockerList.php
@@ -0,0 +1,64 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Workflow;
+
+/**
+ * A list of transition blockers.
+ *
+ * @author Grégoire Pineau <lyrixx@lyrixx.info>
+ */
+final class TransitionBlockerList implements \IteratorAggregate, \Countable
+{
+    private $blockers;
+
+    /**
+     * @param TransitionBlocker[] $blockers
+     */
+    public function __construct(array $blockers = array())
+    {
+        $this->blockers = array();
+
+        foreach ($blockers as $blocker) {
+            $this->add($blocker);
+        }
+    }
+
+    public function add(TransitionBlocker $blocker): void
+    {
+        $this->blockers[] = $blocker;
+    }
+
+    public function reset(): void
+    {
+        $this->blockers = array();
+    }
+
+    public function isEmpty(): bool
+    {
+        return !$this->blockers;
+    }
+
+    /**
+     * {@inheritdoc}
+     *
+     * @return \ArrayIterator|TransitionBlocker[]
+     */
+    public function getIterator()
+    {
+        return new \ArrayIterator($this->blockers);
+    }
+
+    public function count(): int
+    {
+        return \count($this->blockers);
+    }
+}
diff --git a/src/Symfony/Component/Workflow/Workflow.php b/src/Symfony/Component/Workflow/Workflow.php
index ac52794c2d..b31cd12ce7 100644
--- a/src/Symfony/Component/Workflow/Workflow.php
+++ b/src/Symfony/Component/Workflow/Workflow.php
@@ -15,6 +15,8 @@ use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\Workflow\Event\Event;
 use Symfony\Component\Workflow\Event\GuardEvent;
 use Symfony\Component\Workflow\Exception\LogicException;
+use Symfony\Component\Workflow\Exception\NotEnabledTransitionException;
+use Symfony\Component\Workflow\Exception\UndefinedTransitionException;
 use Symfony\Component\Workflow\MarkingStore\MarkingStoreInterface;
 use Symfony\Component\Workflow\MarkingStore\MultipleStateMarkingStore;
 
@@ -85,15 +87,13 @@ class Workflow implements WorkflowInterface
         $marking = $this->getMarking($subject);
 
         foreach ($transitions as $transition) {
-            foreach ($transition->getFroms() as $place) {
-                if (!$marking->has($place)) {
-                    // do not emit guard events for transitions where the marking does not contain
-                    // all "from places" (thus the transition couldn't be applied anyway)
-                    continue 2;
-                }
+            if ($transition->getName() !== $transitionName) {
+                continue;
             }
 
-            if ($transitionName === $transition->getName() && $this->doCan($subject, $marking, $transition)) {
+            $transitionBlockerList = $this->buildTransitionBlockerListForTransition($subject, $marking, $transition);
+
+            if ($transitionBlockerList->isEmpty()) {
                 return true;
             }
         }
@@ -104,19 +104,48 @@ class Workflow implements WorkflowInterface
     /**
      * {@inheritdoc}
      */
-    public function apply($subject, $transitionName)
+    public function buildTransitionBlockerList($subject, string $transitionName): TransitionBlockerList
     {
-        $transitions = $this->getEnabledTransitions($subject);
-
-        // We can shortcut the getMarking method in order to boost performance,
-        // since the "getEnabledTransitions" method already checks the Marking
-        // state
-        $marking = $this->markingStore->getMarking($subject);
-
-        $applied = false;
+        $transitions = $this->definition->getTransitions();
+        $marking = $this->getMarking($subject);
+        $transitionBlockerList = null;
 
         foreach ($transitions as $transition) {
-            if ($transitionName !== $transition->getName()) {
+            if ($transition->getName() !== $transitionName) {
+                continue;
+            }
+
+            $transitionBlockerList = $this->buildTransitionBlockerListForTransition($subject, $marking, $transition);
+
+            if ($transitionBlockerList->isEmpty()) {
+                continue;
+            }
+        }
+
+        if (!$transitionBlockerList) {
+            throw new UndefinedTransitionException($transitionName, $this->name);
+        }
+
+        return $transitionBlockerList;
+    }
+
+    /**
+     * {@inheritdoc}
+     */
+    public function apply($subject, $transitionName)
+    {
+        $marking = $this->getMarking($subject);
+
+        $transitionBlockerList = null;
+        $applied = false;
+
+        foreach ($this->definition->getTransitions() as $transition) {
+            if ($transition->getName() !== $transitionName) {
+                continue;
+            }
+
+            $transitionBlockerList = $this->buildTransitionBlockerListForTransition($subject, $marking, $transition);
+            if (!$transitionBlockerList->isEmpty()) {
                 continue;
             }
 
@@ -137,8 +166,12 @@ class Workflow implements WorkflowInterface
             $this->announce($subject, $transition, $marking);
         }
 
+        if (!$transitionBlockerList) {
+            throw new UndefinedTransitionException($transitionName, $this->name);
+        }
+
         if (!$applied) {
-            throw new LogicException(sprintf('Unable to apply transition "%s" for workflow "%s".', $transitionName, $this->name));
+            throw new NotEnabledTransitionException($transitionName, $this->name, $transitionBlockerList);
         }
 
         return $marking;
@@ -149,16 +182,17 @@ class Workflow implements WorkflowInterface
      */
     public function getEnabledTransitions($subject)
     {
-        $enabled = array();
+        $enabledTransitions = array();
         $marking = $this->getMarking($subject);
 
         foreach ($this->definition->getTransitions() as $transition) {
-            if ($this->doCan($subject, $marking, $transition)) {
-                $enabled[] = $transition;
+            $transitionBlockerList = $this->buildTransitionBlockerListForTransition($subject, $marking, $transition);
+            if ($transitionBlockerList->isEmpty()) {
+                $enabledTransitions[] = $transition;
             }
         }
 
-        return $enabled;
+        return $enabledTransitions;
     }
 
     /**
@@ -185,32 +219,33 @@ class Workflow implements WorkflowInterface
         return $this->markingStore;
     }
 
-    private function doCan($subject, Marking $marking, Transition $transition)
+    private function buildTransitionBlockerListForTransition($subject, Marking $marking, Transition $transition)
     {
         foreach ($transition->getFroms() as $place) {
             if (!$marking->has($place)) {
-                return false;
+                return new TransitionBlockerList(array(
+                    TransitionBlocker::createBlockedByMarking($marking),
+                ));
             }
         }
 
-        if (true === $this->guardTransition($subject, $marking, $transition)) {
-            return false;
+        if (null === $this->dispatcher) {
+            return new TransitionBlockerList();
         }
 
-        return true;
+        $event = $this->guardTransition($subject, $marking, $transition);
+
+        if ($event->isBlocked()) {
+            return $event->getTransitionBlockerList();
+        }
+
+        return new TransitionBlockerList();
     }
 
-    /**
-     * @param object     $subject
-     * @param Marking    $marking
-     * @param Transition $transition
-     *
-     * @return bool|void boolean true if this transition is guarded, ie you cannot use it
-     */
-    private function guardTransition($subject, Marking $marking, Transition $transition)
+    private function guardTransition($subject, Marking $marking, Transition $transition): ?GuardEvent
     {
         if (null === $this->dispatcher) {
-            return;
+            return null;
         }
 
         $event = new GuardEvent($subject, $marking, $transition, $this->name);
@@ -219,10 +254,10 @@ class Workflow implements WorkflowInterface
         $this->dispatcher->dispatch(sprintf('workflow.%s.guard', $this->name), $event);
         $this->dispatcher->dispatch(sprintf('workflow.%s.guard.%s', $this->name, $transition->getName()), $event);
 
-        return $event->isBlocked();
+        return $event;
     }
 
-    private function leave($subject, Transition $transition, Marking $marking)
+    private function leave($subject, Transition $transition, Marking $marking): void
     {
         $places = $transition->getFroms();
 
@@ -242,7 +277,7 @@ class Workflow implements WorkflowInterface
         }
     }
 
-    private function transition($subject, Transition $transition, Marking $marking)
+    private function transition($subject, Transition $transition, Marking $marking): void
     {
         if (null === $this->dispatcher) {
             return;
@@ -255,7 +290,7 @@ class Workflow implements WorkflowInterface
         $this->dispatcher->dispatch(sprintf('workflow.%s.transition.%s', $this->name, $transition->getName()), $event);
     }
 
-    private function enter($subject, Transition $transition, Marking $marking)
+    private function enter($subject, Transition $transition, Marking $marking): void
     {
         $places = $transition->getTos();
 
@@ -275,7 +310,7 @@ class Workflow implements WorkflowInterface
         }
     }
 
-    private function entered($subject, Transition $transition, Marking $marking)
+    private function entered($subject, Transition $transition, Marking $marking): void
     {
         if (null === $this->dispatcher) {
             return;
@@ -291,7 +326,7 @@ class Workflow implements WorkflowInterface
         }
     }
 
-    private function completed($subject, Transition $transition, Marking $marking)
+    private function completed($subject, Transition $transition, Marking $marking): void
     {
         if (null === $this->dispatcher) {
             return;
@@ -304,7 +339,7 @@ class Workflow implements WorkflowInterface
         $this->dispatcher->dispatch(sprintf('workflow.%s.completed.%s', $this->name, $transition->getName()), $event);
     }
 
-    private function announce($subject, Transition $initialTransition, Marking $marking)
+    private function announce($subject, Transition $initialTransition, Marking $marking): void
     {
         if (null === $this->dispatcher) {
             return;
diff --git a/src/Symfony/Component/Workflow/WorkflowInterface.php b/src/Symfony/Component/Workflow/WorkflowInterface.php
index 15ee8a4ec8..2460963cba 100644
--- a/src/Symfony/Component/Workflow/WorkflowInterface.php
+++ b/src/Symfony/Component/Workflow/WorkflowInterface.php
@@ -37,11 +37,16 @@ interface WorkflowInterface
      * @param string $transitionName A transition
      *
      * @return bool true if the transition is enabled
-     *
-     * @throws LogicException
      */
     public function can($subject, $transitionName);
 
+    /**
+     * Builds a TransitionBlockerList to know why a transition is blocked.
+     *
+     * @param object $subject A subject
+     */
+    public function buildTransitionBlockerList($subject, string $transitionName): TransitionBlockerList;
+
     /**
      * Fire a transition.
      *
@@ -51,7 +56,6 @@ interface WorkflowInterface
      * @return Marking The new Marking
      *
      * @throws LogicException If the transition is not applicable
-     * @throws LogicException If the transition does not exist
      */
     public function apply($subject, $transitionName);