diff --git a/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php b/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php
index 9688add..d8651e6 100644
--- a/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php
+++ b/core/lib/Drupal/Component/Plugin/DefaultPluginBag.php
@@ -7,7 +7,7 @@
 
 namespace Drupal\Component\Plugin;
 
-use Drupal\Component\Plugin\Exception\UnknownPluginException;
+use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 use Drupal\Component\Utility\MapArray;
 
 /**
@@ -75,7 +75,7 @@ public function __construct(PluginManagerInterface $manager, array $configuratio
   protected function initializePlugin($instance_id) {
     $configuration = isset($this->configurations[$instance_id]) ? $this->configurations[$instance_id] : array();
     if (!isset($configuration[$this->pluginKey])) {
-      throw new UnknownPluginException($instance_id);
+      throw new PluginNotFoundException($instance_id);
     }
     $this->set($instance_id, $this->manager->createInstance($configuration[$this->pluginKey], $configuration));
   }
diff --git a/core/lib/Drupal/Component/Plugin/Exception/UnknownPluginException.php b/core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php
similarity index 34%
rename from core/lib/Drupal/Component/Plugin/Exception/UnknownPluginException.php
rename to core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php
index 59586a5..f2621a8 100644
--- a/core/lib/Drupal/Component/Plugin/Exception/UnknownPluginException.php
+++ b/core/lib/Drupal/Component/Plugin/Exception/PluginNotFoundException.php
@@ -1,7 +1,8 @@
 <?php
+
 /**
  * @file
- * Contains \Drupal\Component\Plugin\Exception\UnknownPluginException.
+ * Contains \Drupal\Component\Plugin\Exception\PluginNotFoundException.
  */
 
 namespace Drupal\Component\Plugin\Exception;
@@ -9,21 +10,21 @@
 use Drupal\Component\Utility\String;
 
 /**
- * Plugin exception class to be thrown when a nonexistent plugin was requested.
+ * Plugin exception class to be thrown when a plugin ID could not be found.
  */
-class UnknownPluginException extends PluginException {
+class PluginNotFoundException extends PluginException {
 
   /**
-   * Construct an UnknownPluginException exception.
+   * Construct an PluginNotFoundException exception.
    *
-   * @param string $instance_id
-   *   The invalid instance ID that failed.
+   * @param string $plugin_id
+   *   The plugin ID that was not found.
    *
    * @see \Exception for remaining parameters.
    */
-  public function __construct($instance_id, $message = '', $code = 0, \Exception $previous = NULL) {
+  public function __construct($plugin_id, $message = '', $code = 0, \Exception $previous = NULL) {
     if (empty($message)) {
-      $message = String::format("Unknown plugin ID '@instance'.", array('@instance' => $instance_id));
+      $message = String::format("Plugin ID '@instance' was not found.", array('@instance' => $plugin_id));
     }
     parent::__construct($message, $code, $previous);
   }
diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index 8f7e929..e2aac3e 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -8,7 +8,7 @@
 namespace Drupal\Core\Entity;
 
 use Drupal\Component\Plugin\Exception\InvalidPluginDefinitionException;
-use Drupal\Component\Plugin\Exception\UnknownPluginException;
+use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 use Drupal\Component\Plugin\PluginManagerBase;
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Utility\String;
@@ -170,7 +170,7 @@ public function getDefinition($entity_type_id, $exception_on_invalid = FALSE) {
       return NULL;
     }
 
-    throw new UnknownPluginException($entity_type_id, sprintf('The "%s" entity type does not exist.', $entity_type_id));
+    throw new PluginNotFoundException($entity_type_id, sprintf('The "%s" entity type does not exist.', $entity_type_id));
   }
 
   /**
diff --git a/core/modules/image/image.module b/core/modules/image/image.module
index 3751b8c..1f5d8a5 100644
--- a/core/modules/image/image.module
+++ b/core/modules/image/image.module
@@ -6,7 +6,7 @@
  */
 
 use Drupal\Core\Entity\EntityInterface;
-use Drupal\Component\Plugin\Exception\UnknownPluginException;
+use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 use Drupal\field\Entity\Field;
 use Drupal\field\Entity\FieldInstance;
 use Drupal\file\Entity\File;
@@ -76,7 +76,7 @@ function image_help($path, $arg) {
       try {
         $effect = entity_load('image_style', $arg[5])->getEffect($arg[7]);
       }
-      catch (UnknownPluginException $e) {
+      catch (PluginNotFoundException $e) {
         return NULL;
       }
       $effect_definition = $effect->getPluginDefinition();
diff --git a/core/modules/image/lib/Drupal/image/Form/ImageEffectFormBase.php b/core/modules/image/lib/Drupal/image/Form/ImageEffectFormBase.php
index 5323592..100b9ce 100644
--- a/core/modules/image/lib/Drupal/image/Form/ImageEffectFormBase.php
+++ b/core/modules/image/lib/Drupal/image/Form/ImageEffectFormBase.php
@@ -10,7 +10,7 @@
 use Drupal\Core\Form\FormBase;
 use Drupal\image\ConfigurableImageEffectInterface;
 use Drupal\image\ImageStyleInterface;
-use Drupal\Component\Plugin\Exception\UnknownPluginException;
+use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 use Drupal\Component\Utility\String;
 use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
 
@@ -58,7 +58,7 @@ public function buildForm(array $form, array &$form_state, ImageStyleInterface $
     try {
       $this->imageEffect = $this->prepareImageEffect($image_effect);
     }
-    catch (UnknownPluginException $e) {
+    catch (PluginNotFoundException $e) {
       throw new NotFoundHttpException(String::format("Invalid effect id: '@id'.", array('@id' => $image_effect)));
     }
     $request = $this->getRequest();
diff --git a/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php b/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
index f12c146..b2aea4e 100644
--- a/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
+++ b/core/tests/Drupal/Tests/Component/Plugin/DefaultPluginBagTest.php
@@ -59,7 +59,8 @@ public function testGet() {
   /**
    * Tests the get method with an non existing plugin ID.
    *
-   * @expectedException \Drupal\Component\Plugin\Exception\PluginException
+   * @expectedException \Drupal\Component\Plugin\Exception\PluginNotFoundException
+   * @expectedExceptionMessage Plugin ID 'pear' was not found.
    */
   public function testGetNotExistingPlugin() {
     $this->setupPluginBag();
diff --git a/core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php b/core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php
index 24df3b7..9ec433d 100644
--- a/core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php
+++ b/core/tests/Drupal/Tests/Core/Entity/EntityManagerTest.php
@@ -223,7 +223,8 @@ public function providerTestGetDefinition() {
    *
    * @covers ::getDefinition()
    *
-   * @expectedException \Drupal\Component\Plugin\Exception\UnknownPluginException
+   * @expectedException \Drupal\Component\Plugin\Exception\PluginNotFoundException
+   * @expectedExceptionMessage The "pear" entity type does not exist.
    */
   public function testGetDefinitionInvalidException() {
     $this->setUpEntityManager();