diff --git a/entity_browser.api.php b/entity_browser.api.php
index 8231aea..230eefc 100644
--- a/entity_browser.api.php
+++ b/entity_browser.api.php
@@ -62,5 +62,16 @@ function hook_entity_browser_field_widget_display_info_alter(&$field_displays) {
 }
 
 /**
+ * Alter the information provided in \Drupal\entity_browser\Annotation\EntityBrowserWidgetValidation.
+ *
+ * @param $validation_plugins
+ *   The array of widget validation plugins, keyed on the machine-readable
+ *   name.
+ */
+function hook_entity_browser_widget_validation_info_alter(&$validation_plugins) {
+  $field_displays['not_null']['label'] = t('Not null fabulous validator');
+}
+
+/**
  * @} End of "addtogroup hooks".
  */
diff --git a/entity_browser.services.yml b/entity_browser.services.yml
index 88fedfd..4c27024 100644
--- a/entity_browser.services.yml
+++ b/entity_browser.services.yml
@@ -14,6 +14,9 @@ services:
   plugin.manager.entity_browser.field_widget_display:
     class: Drupal\entity_browser\FieldWidgetDisplayManager
     parent: default_plugin_manager
+  plugin.manager.entity_browser.widget_validation:
+    class: Drupal\entity_browser\WidgetValidationManager
+    parent: default_plugin_manager
   entity_browser.route_subscriber:
     class: Drupal\entity_browser\RouteSubscriber
     arguments: ['@entity.manager', '@plugin.manager.entity_browser.display', '@entity.query']
diff --git a/src/Annotation/EntityBrowserWidgetValidation.php b/src/Annotation/EntityBrowserWidgetValidation.php
new file mode 100644
index 0000000..26038e2
--- /dev/null
+++ b/src/Annotation/EntityBrowserWidgetValidation.php
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity_browser\Annotation\EntityBrowserWidgetValidation.
+ */
+
+namespace Drupal\entity_browser\Annotation;
+
+use Drupal\Component\Annotation\Plugin;
+
+/**
+ * Defines an entity browser widget validation annotation object.
+ *
+ * @see hook_entity_browser_widget_validation_info_alter()
+ *
+ * @Annotation
+ */
+class EntityBrowserWidgetValidation extends Plugin {
+
+  /**
+   * The plugin ID.
+   *
+   * @var string
+   */
+  public $id;
+
+  /**
+   * The human-readable name of the widget validator.
+   *
+   * @ingroup plugin_translatable
+   *
+   * @var \Drupal\Core\Annotation\Translation
+   */
+  public $label;
+
+  /**
+   * The constraint ID.
+   *
+   * @var string
+   */
+  public $constraint;
+}
diff --git a/src/DisplayBase.php b/src/DisplayBase.php
index f2c2569..8b455be 100644
--- a/src/DisplayBase.php
+++ b/src/DisplayBase.php
@@ -3,6 +3,7 @@
 namespace Drupal\entity_browser;
 
 use Drupal\Component\Uuid\UuidInterface;
+use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Symfony\Component\DependencyInjection\ContainerInterface;
@@ -51,6 +52,13 @@ abstract class DisplayBase extends PluginBase implements DisplayInterface, Conta
   protected $uuid = NULL;
 
   /**
+   * Key value store.
+   *
+   * @var \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface
+   */
+  protected $keyValue;
+
+  /**
    * Constructs display plugin.
    *
    * @param array $configuration
@@ -63,12 +71,15 @@ abstract class DisplayBase extends PluginBase implements DisplayInterface, Conta
    *   Event dispatcher service.
    * @param \Drupal\Component\Uuid\UuidInterface
    *   UUID generator interface.
+   * @param \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface $key_value
+   *   The key value store.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, UuidInterface $uuid_generator) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, UuidInterface $uuid_generator, KeyValueStoreExpirableInterface $key_value) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
     $this->configuration += $this->defaultConfiguration();
     $this->eventDispatcher = $event_dispatcher;
     $this->uuidGenerator = $uuid_generator;
+    $this->keyValue = $key_value;
   }
 
   /**
@@ -80,7 +91,8 @@ abstract class DisplayBase extends PluginBase implements DisplayInterface, Conta
       $plugin_id,
       $plugin_definition,
       $container->get('event_dispatcher'),
-      $container->get('uuid')
+      $container->get('uuid'),
+      $container->get('keyvalue.expirable')->get('entity_browser')
     );
   }
 
@@ -139,4 +151,24 @@ abstract class DisplayBase extends PluginBase implements DisplayInterface, Conta
     $this->uuid = $uuid;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function setValidators(array $validators) {
+    // Generate the hash that we use as key for the key/value.
+    $hash = md5(serialize($validators));
+
+    if (!$this->keyValue->has($hash)) {
+      $this->keyValue->set($hash, $validators);
+    }
+    return $hash;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getValidators($hash) {
+    return $this->keyValue->get($hash, []);
+  }
+
 }
diff --git a/src/DisplayInterface.php b/src/DisplayInterface.php
index a73f443..e0238d2 100644
--- a/src/DisplayInterface.php
+++ b/src/DisplayInterface.php
@@ -36,11 +36,14 @@ interface DisplayInterface extends PluginInspectionInterface, ConfigurablePlugin
    *
    * @param \Drupal\Core\Form\FormStateInterface $form_state
    *   The form state object.
+   * @param string $validators
+   *   (optional) Validators hash identifier as returned by
+   *   EntityReference::prepareValidators().
    *
    * @return array
    *   An array suitable for drupal_render().
    */
-  public function displayEntityBrowser(FormStateInterface $form_state);
+  public function displayEntityBrowser(FormStateInterface $form_state, $validators = '');
 
   /**
    * Indicates completed selection.
@@ -69,4 +72,29 @@ interface DisplayInterface extends PluginInspectionInterface, ConfigurablePlugin
    */
   public function setUuid($uuid);
 
+  /**
+   * Set validators.
+   *
+   * Saves Entity Browser Widget validators in key/value storage if an identical
+   * set of constraints is not already stored there.
+   *
+   * @param array $validators
+   *   An array where keys are validator ids and values configurations for them.
+   *
+   * @return string
+   *   The hash generated from hashing the validators array.
+   */
+  public function setValidators(array $validators);
+
+  /**
+   * Get validators.
+   *
+   * @param $hash
+   *   The hash generated from hashing the validators array.
+   *
+   * @return mixed
+   *   An array where keys are validator ids and values configurations for them
+   *   or empty array if no validators are stored.
+   */
+  public function getValidators($hash);
 }
diff --git a/src/Plugin/EntityBrowser/Display/IFrame.php b/src/Plugin/EntityBrowser/Display/IFrame.php
index 9eed34f..df86073 100644
--- a/src/Plugin/EntityBrowser/Display/IFrame.php
+++ b/src/Plugin/EntityBrowser/Display/IFrame.php
@@ -5,6 +5,7 @@ namespace Drupal\entity_browser\Plugin\EntityBrowser\Display;
 use Drupal\Component\Uuid\UuidInterface;
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Url;
 use Drupal\entity_browser\DisplayBase;
@@ -72,9 +73,11 @@ class IFrame extends DisplayBase implements DisplayRouterInterface {
    *   Current request.
    * @param \Drupal\Core\Path\CurrentPathStack $current_path
    *   The current path.
+   * @param \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface $key_value
+   *   The key value store.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, UuidInterface $uuid, RouteMatchInterface $current_route_match, Request $request, CurrentPathStack $current_path) {
-    parent::__construct($configuration, $plugin_id, $plugin_definition, $event_dispatcher, $uuid);
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, UuidInterface $uuid, RouteMatchInterface $current_route_match, Request $request, CurrentPathStack $current_path, KeyValueStoreExpirableInterface $key_value) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition, $event_dispatcher, $uuid, $key_value);
     $this->currentRouteMatch = $current_route_match;
     $this->request = $request;
     $this->currentPath = $current_path;
@@ -89,10 +92,12 @@ class IFrame extends DisplayBase implements DisplayRouterInterface {
       $plugin_id,
       $plugin_definition,
       $container->get('event_dispatcher'),
+      $container->get('current_route_match'),
       $container->get('uuid'),
       $container->get('current_route_match'),
       $container->get('request_stack')->getCurrentRequest(),
-      $container->get('path.current')
+      $container->get('path.current'),
+      $container->get('keyvalue.expirable')->get('entity_browser')
     );
   }
 
@@ -111,7 +116,7 @@ class IFrame extends DisplayBase implements DisplayRouterInterface {
   /**
    * {@inheritdoc}
    */
-  public function displayEntityBrowser(FormStateInterface $form_state) {
+  public function displayEntityBrowser(FormStateInterface $form_state, $validators = '') {
     $uuid = $this->getUuid();
     /** @var \Drupal\entity_browser\Events\RegisterJSCallbacks $event */
     $js_event_object = new RegisterJSCallbacks($this->configuration['entity_browser_id'], $uuid);
@@ -123,6 +128,7 @@ class IFrame extends DisplayBase implements DisplayRouterInterface {
         'query' => [
           'uuid' => $uuid,
           'original_path' => $original_path,
+          'validators' => $validators,
         ],
       ],
       'attributes' => [
diff --git a/src/Plugin/EntityBrowser/Display/Modal.php b/src/Plugin/EntityBrowser/Display/Modal.php
index 433cf27..6941d94 100644
--- a/src/Plugin/EntityBrowser/Display/Modal.php
+++ b/src/Plugin/EntityBrowser/Display/Modal.php
@@ -7,6 +7,7 @@ use Drupal\Component\Utility\NestedArray;
 use Drupal\Component\Uuid\UuidInterface;
 use Drupal\Core\Ajax\OpenDialogCommand;
 use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Url;
 use Drupal\entity_browser\DisplayBase;
@@ -73,13 +74,15 @@ class Modal extends DisplayBase implements DisplayRouterInterface {
    *   UUID generator interface.
    * @param \Drupal\Core\Routing\RouteMatchInterface
    *   The currently active route match object.
-   * @param \Symfony\Component\HttpFoundation\Request $request
-   *   Current request.
    * @param \Drupal\Core\Path\CurrentPathStack $current_path
    *   The current path.
+   * @param \Symfony\Component\HttpFoundation\Request $request
+   *   Current request.
+   * @param \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface $key_value
+   *   The key value store.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, UuidInterface $uuid, RouteMatchInterface $current_route_match, CurrentPathStack $current_path, Request $request) {
-    parent::__construct($configuration, $plugin_id, $plugin_definition, $event_dispatcher, $uuid);
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, UuidInterface $uuid, RouteMatchInterface $current_route_match, CurrentPathStack $current_path, Request $request, KeyValueStoreExpirableInterface $key_value) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition, $event_dispatcher, $uuid, $key_value);
     $this->currentRouteMatch = $current_route_match;
     $this->currentPath = $current_path;
     $this->request = $request;
@@ -97,7 +100,8 @@ class Modal extends DisplayBase implements DisplayRouterInterface {
       $container->get('uuid'),
       $container->get('current_route_match'),
       $container->get('path.current'),
-      $container->get('request_stack')->getCurrentRequest()
+      $container->get('request_stack')->getCurrentRequest(),
+      $container->get('keyvalue.expirable')->get('entity_browser')
     );
   }
 
@@ -115,7 +119,7 @@ class Modal extends DisplayBase implements DisplayRouterInterface {
   /**
    * {@inheritdoc}
    */
-  public function displayEntityBrowser(FormStateInterface $form_state) {
+  public function displayEntityBrowser(FormStateInterface $form_state, $validators = '') {
     $uuid = $this->getUuid();
     $js_event_object = new RegisterJSCallbacks($this->configuration['entity_browser_id'], $uuid);
     $js_event_object->registerCallback('Drupal.entityBrowser.selectionCompleted');
@@ -126,6 +130,7 @@ class Modal extends DisplayBase implements DisplayRouterInterface {
         'query' => [
           'uuid' => $uuid,
           'original_path' => $original_path,
+          'validators' => $validators,
         ],
       ],
       'attributes' => [
diff --git a/src/Plugin/EntityBrowser/Display/Standalone.php b/src/Plugin/EntityBrowser/Display/Standalone.php
index 9b8fbc6..949fc5e 100644
--- a/src/Plugin/EntityBrowser/Display/Standalone.php
+++ b/src/Plugin/EntityBrowser/Display/Standalone.php
@@ -45,7 +45,7 @@ class Standalone extends DisplayBase implements DisplayRouterInterface {
   /**
    * {@inheritdoc}
    */
-  public function displayEntityBrowser(FormStateInterface $form_state) {
+  public function displayEntityBrowser(FormStateInterface $form_state, $validators = '') {
     // @TODO Implement it.
   }
 
diff --git a/src/Plugin/EntityBrowser/Widget/Upload.php b/src/Plugin/EntityBrowser/Widget/Upload.php
index eff51b2..f882dba 100644
--- a/src/Plugin/EntityBrowser/Widget/Upload.php
+++ b/src/Plugin/EntityBrowser/Widget/Upload.php
@@ -103,13 +103,12 @@ class Upload extends WidgetBase {
   /**
    * {@inheritdoc}
    */
-  public function validate(array &$form, FormStateInterface $form_state) {
-    $uploaded_files = $form_state->getValue(['upload'], []);
-    $trigger = $form_state->getTriggeringElement();
-    // Only validate if we are uploading a file.
-    if (empty($uploaded_files)  && $trigger['#value'] == 'Upload') {
-      $form_state->setError($form['widget']['upload'], t('At least one file should be uploaded.'));
+  public function prepareEntities(FormStateInterface $form_state) {
+    $files = [];
+    foreach ($form_state->getValue(['upload'], []) as $fid) {
+      $files[] = $this->entityManager->getStorage('file')->load($fid);
     }
+    return $files;
   }
 
   /**
diff --git a/src/Plugin/EntityBrowser/Widget/View.php b/src/Plugin/EntityBrowser/Widget/View.php
index 6e86429..98a4849 100644
--- a/src/Plugin/EntityBrowser/Widget/View.php
+++ b/src/Plugin/EntityBrowser/Widget/View.php
@@ -4,6 +4,7 @@ namespace Drupal\entity_browser\Plugin\EntityBrowser\Widget;
 
 use Drupal\Component\Plugin\Exception\PluginNotFoundException;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
 use Drupal\Core\Render\Element;
 use Drupal\entity_browser\WidgetBase;
 use Drupal\Core\Url;
@@ -13,6 +14,7 @@ use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Drupal\Core\Entity\EntityManagerInterface;
+use Symfony\Component\HttpFoundation\Request;
 
 /**
  * Uses a view to provide entity listing in a browser's widget.
@@ -53,7 +55,9 @@ class View extends WidgetBase implements ContainerFactoryPluginInterface {
       $plugin_definition,
       $container->get('event_dispatcher'),
       $container->get('entity.manager'),
-      $container->get('current_user')
+      $container->get('current_user'),
+      $container->get('keyvalue.expirable')->get('entity_browser'),
+      $container->get('request_stack')->getCurrentRequest()
     );
   }
 
@@ -71,9 +75,9 @@ class View extends WidgetBase implements ContainerFactoryPluginInterface {
    * @param \Drupal\Core\Session\AccountInterface $current_user
    *   The current user.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, EntityManagerInterface $entity_manager, AccountInterface $current_user) {
-    parent::__construct($configuration, $plugin_id, $plugin_definition, $event_dispatcher, $entity_manager);
-    $this->currentUser = $current_user;
+   public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, EntityManagerInterface $entity_manager, AccountInterface $current_user, KeyValueStoreExpirableInterface $key_value, Request $request) {
+     parent::__construct($configuration, $plugin_id, $plugin_definition, $event_dispatcher, $entity_manager, $key_value, $request);
+     $this->currentUser = $current_user;
   }
 
   /**
diff --git a/src/Plugin/EntityBrowser/WidgetValidation/Cardinality.php b/src/Plugin/EntityBrowser/WidgetValidation/Cardinality.php
new file mode 100644
index 0000000..c688853
--- /dev/null
+++ b/src/Plugin/EntityBrowser/WidgetValidation/Cardinality.php
@@ -0,0 +1,39 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity_browser\Plugin\EntityBrowser\WidgetValidation\Cardinality.
+ */
+
+namespace Drupal\entity_browser\Plugin\EntityBrowser\WidgetValidation;
+
+use Drupal\Core\TypedData\ListDataDefinition;
+use Drupal\entity_browser\WidgetValidationBase;
+
+/**
+ * Validates that the widget returns the appropriate number of elements.
+ *
+ * @EntityBrowserWidgetValidation(
+ *   id = "cardinality",
+ *   label = @Translation("Not empty validator"),
+ *   constraint = "Count"
+ * )
+ */
+class Cardinality extends WidgetValidationBase {
+  /**
+   * {@inheritdoc}
+   */
+  public function validate(array $entities, $options = []) {
+    $data_definition = ListDataDefinition::create('integer');
+    $plugin_definition = $this->getPluginDefinition();
+    $data_definition->addConstraint($plugin_definition['constraint'], $options);
+
+    $ids = [];
+    foreach ($entities as $entity) {
+      $ids[] = $entity->id();
+    }
+
+    $typed_data = \Drupal::typedDataManager()->create($data_definition, $ids);
+    return $typed_data->validate();
+  }
+}
diff --git a/src/Plugin/EntityBrowser/WidgetValidation/EntityType.php b/src/Plugin/EntityBrowser/WidgetValidation/EntityType.php
new file mode 100644
index 0000000..588a89f
--- /dev/null
+++ b/src/Plugin/EntityBrowser/WidgetValidation/EntityType.php
@@ -0,0 +1,38 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity_browser\Plugin\EntityBrowser\WidgetValidation\EntityType.
+ */
+
+namespace Drupal\entity_browser\Plugin\EntityBrowser\WidgetValidation;
+
+use Drupal\entity_browser\WidgetValidationBase;
+use Symfony\Component\Validator\ConstraintViolationList;
+
+/**
+ * Validates that the widget returns the appropriate number of elements.
+ *
+ * @EntityBrowserWidgetValidation(
+ *   id = "entity_type",
+ *   label = @Translation("Entity type validator"),
+ * )
+ */
+class EntityType extends WidgetValidationBase {
+  /**
+   * {@inheritdoc}
+   */
+  public function validate(array $entities, $options = []) {
+    $data_definition = \Drupal::typedDataManager()->createDataDefinition('entity_reference');
+    $plugin_definition = $this->getPluginDefinition();
+    $data_definition->addConstraint($plugin_definition['constraint'], $options);
+
+    $violations = new ConstraintViolationList([]);
+    foreach ($entities as $entity) {
+      $validation_result = \Drupal::typedDataManager()->create($data_definition, $entity)->validate();
+      $violations->addAll($validation_result);
+    }
+
+    return $violations;
+  }
+}
diff --git a/src/Plugin/Field/FieldWidget/EntityReference.php b/src/Plugin/Field/FieldWidget/EntityReference.php
index 1f8520d..f2164d8 100644
--- a/src/Plugin/Field/FieldWidget/EntityReference.php
+++ b/src/Plugin/Field/FieldWidget/EntityReference.php
@@ -12,6 +12,7 @@ use Drupal\Core\Field\FieldItemListInterface;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\Core\Field\WidgetBase;
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Url;
 use Drupal\Core\Validation\Plugin\Validation\Constraint\NotNullConstraint;
@@ -50,6 +51,11 @@ class EntityReference extends WidgetBase implements ContainerFactoryPluginInterf
   protected $fieldDisplayManager;
 
   /**
+   * @var \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface
+   */
+  protected $keyValue;
+
+  /**
    * The depth of the delete button.
    *
    * This property exists so it can be changed if subclasses.
@@ -77,11 +83,14 @@ class EntityReference extends WidgetBase implements ContainerFactoryPluginInterf
    *   Event dispatcher.
    * @param \Drupal\entity_browser\FieldWidgetDisplayManager $field_display_manager
    *   Field widget display plugin manager.
+   * @param \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface $key_value
+   *   The key value store.
    */
-  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, EntityManagerInterface $entity_manager, EventDispatcherInterface $event_dispatcher, FieldWidgetDisplayManager $field_display_manager) {
+  public function __construct($plugin_id, $plugin_definition, FieldDefinitionInterface $field_definition, array $settings, array $third_party_settings, EntityManagerInterface $entity_manager, EventDispatcherInterface $event_dispatcher, FieldWidgetDisplayManager $field_display_manager, KeyValueStoreExpirableInterface $key_value) {
     parent::__construct($plugin_id, $plugin_definition, $field_definition, $settings, $third_party_settings);
     $this->entityManager = $entity_manager;
     $this->fieldDisplayManager = $field_display_manager;
+    $this->keyValue = $key_value;
   }
 
   /**
@@ -96,7 +105,8 @@ class EntityReference extends WidgetBase implements ContainerFactoryPluginInterf
       $configuration['third_party_settings'],
       $container->get('entity.manager'),
       $container->get('event_dispatcher'),
-      $container->get('plugin.manager.entity_browser.field_widget_display')
+      $container->get('plugin.manager.entity_browser.field_widget_display'),
+      $container->get('keyvalue.expirable')->get('entity_browser')
     );
   }
 
@@ -375,12 +385,20 @@ class EntityReference extends WidgetBase implements ContainerFactoryPluginInterf
       ],
     ];
 
+
+    // Gather and set validators.
+    // @todo Is there a better place to do that?
     $cardinality = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();
+    $validators = $this->prepareValidators([
+      'cardinality' => ['min' => $cardinality],
+      'entity_type' => ['type' => $entity_type],
+    ]);
+
     if ($cardinality == FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED || count($ids) < $cardinality) {
       $entity_browser_uuid = sha1(implode('-', array_merge($form['#parents'], [$this->fieldDefinition->getName(), $delta])));
       $entity_browser_display = $entity_browser->getDisplay();
       $entity_browser_display->setUuid($entity_browser_uuid);
-      $element['entity_browser'] = $entity_browser_display->displayEntityBrowser($form_state);
+      $element['entity_browser'] = $entity_browser_display->displayEntityBrowser($form_state, $validators);
       $element['#attached']['library'][] = 'entity_browser/entity_reference';
       $element['#attached']['drupalSettings']['entity_browser'] = [
         $entity_browser->getDisplay()->getUuid() => [
@@ -540,4 +558,40 @@ class EntityReference extends WidgetBase implements ContainerFactoryPluginInterf
     ];
   }
 
+  /**
+   * Prepare validators.
+   *
+   * Saves Entity Browser Widget validators in key/value storage if an identical
+   * set of constraints is not already stored there.
+   *
+   * @param array $validators
+   *   An array where keys are validator ids and values configurations for them.
+   *
+   * @return string
+   *   The hash generated from hashing the validators array.
+   */
+  public function prepareValidators(array $validators) {
+    // Generate the hash that we use as key for the key/value.
+    $hash = md5(serialize($validators));
+
+    if (!$this->keyValue->has($hash)) {
+      $this->keyValue->set($hash, $validators);
+    }
+
+    return $hash;
+  }
+
+  /**
+   * Get validators.
+   *
+   * @param \Drupal\entity_browser\string $hash
+   *   The hash generated from hashing the validators array.
+   *
+   * @return mixed
+   *   An array where keys are validator ids and values configurations for them
+   *   or empty array if no validators are stored.
+   */
+  public function getValidators($hash) {
+    return $this->keyValue->get($hash, []);
+  }
 }
diff --git a/src/WidgetBase.php b/src/WidgetBase.php
index e1d702d..f35fe19 100644
--- a/src/WidgetBase.php
+++ b/src/WidgetBase.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\entity_browser;
 
+use Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Form\FormStateInterface;
@@ -10,6 +11,7 @@ use Drupal\entity_browser\Events\EntitySelectionEvent;
 use Drupal\entity_browser\Events\Events;
 use Symfony\Component\DependencyInjection\ContainerInterface;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
+use Symfony\Component\HttpFoundation\Request;
 
 /**
  * Base class for widget plugins.
@@ -60,6 +62,20 @@ abstract class WidgetBase extends PluginBase implements WidgetInterface, Contain
   protected $entityManager;
 
   /**
+   * The Expirable key value store.
+   *
+   * @var \Drupal\Core\KeyValueStore\KeyValueStoreExpirableInterface
+   */
+  private $keyValue;
+
+  /**
+   * The Request object.
+   *
+   * @var \Symfony\Component\HttpFoundation\Request
+   */
+  private $request;
+
+  /**
    * Constructs widget plugin.
    *
    * @param array $configuration
@@ -71,11 +87,13 @@ abstract class WidgetBase extends PluginBase implements WidgetInterface, Contain
    * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
    *   Event dispatcher service.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, EntityManagerInterface $entity_manager) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, EventDispatcherInterface $event_dispatcher, EntityManagerInterface $entity_manager, KeyValueStoreExpirableInterface $key_value, Request $request) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
     $this->eventDispatcher = $event_dispatcher;
     $this->entityManager = $entity_manager;
     $this->setConfiguration($configuration);
+    $this->keyValue = $key_value;
+    $this->request = $request;
   }
 
   /**
@@ -87,7 +105,9 @@ abstract class WidgetBase extends PluginBase implements WidgetInterface, Contain
       $plugin_id,
       $plugin_definition,
       $container->get('event_dispatcher'),
-      $container->get('entity.manager')
+      $container->get('entity.manager'),
+      $container->get('keyvalue.expirable')->get('entity_browser'),
+      $container->get('request_stack')->getCurrentRequest()
     );
   }
 
@@ -187,7 +207,51 @@ abstract class WidgetBase extends PluginBase implements WidgetInterface, Contain
   /**
    * {@inheritdoc}
    */
-  public function validate(array &$form, FormStateInterface $form_state) {}
+  public function prepareEntities(FormStateInterface $form_state) {
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate(array &$form, FormStateInterface $form_state) {
+    $trigger = $form_state->getTriggeringElement();
+
+    if (in_array('submit', $trigger['#array_parents'])) {
+      $entities = $this->prepareEntities($form_state);
+      $violations = $this->runWidgetValidators($entities);
+      if (!empty($violations)) {
+        /** @var \Symfony\Component\Validator\ConstraintViolationListInterface $violation */
+        foreach ($violations as $violation) {
+          $form_state->setError($form['widget']['upload'], $violation->getMessage());
+        }
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function runWidgetValidators(array $entities, $validators = []) {
+    // @todo Implement a centralized way to get arguments from path for all widgets?
+    if ($validators_hash = $this->request->get('validators')) {
+      $passed_validators = $this->keyValue->get($validators_hash);
+
+      if (!empty($passed_validators)) {
+        $validators += $passed_validators;
+      }
+    }
+
+    foreach ($validators as $validator_id => $options) {
+      /** @var \Drupal\entity_browser\WidgetValidationInterface $widget_validator */
+      $widget_validator = \Drupal::service('plugin.manager.entity_browser.widget_validation')->createInstance($validator_id, []);
+    }
+
+    if ($widget_validator) {
+      return $widget_validator->validate($entities, $options);
+    }
+    return [];
+  }
 
   /**
    * {@inheritdoc}
diff --git a/src/WidgetInterface.php b/src/WidgetInterface.php
index ddd3763..0146e7f 100644
--- a/src/WidgetInterface.php
+++ b/src/WidgetInterface.php
@@ -88,6 +88,20 @@ interface WidgetInterface extends PluginInspectionInterface, ConfigurablePluginI
   public function getForm(array &$original_form, FormStateInterface $form_state, array $aditional_widget_parameters);
 
   /**
+   * Prepares the entities without saving them.
+   *
+   * We need this method when we want to validation or perform other operations
+   * before submit.
+   *
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The form state object.
+   *
+   * @return \Drupal\Core\Entity\ContentEntityInterface[]
+   *   Array of entities.
+   */
+  public function prepareEntities(FormStateInterface $form_state);
+
+  /**
    * Validates form.
    *
    * @param array $form
@@ -98,6 +112,20 @@ interface WidgetInterface extends PluginInspectionInterface, ConfigurablePluginI
   public function validate(array &$form, FormStateInterface $form_state);
 
   /**
+   * Run widget validators.
+   *
+   * @param array $entities
+   *   Array of entity ids to validate.
+   * @param array $validators
+   *   Array of additional widget validator ids.
+   *
+   * @return \Symfony\Component\Validator\ConstraintViolationListInterface
+   *   A list of constraint violations. If the list is empty, validation
+   *   succeeded.
+   */
+  public function runWidgetValidators(array $entities, $validators = []);
+
+  /**
    * Submits form.
    *
    * @param array $element
diff --git a/src/WidgetValidationBase.php b/src/WidgetValidationBase.php
new file mode 100644
index 0000000..5200e58
--- /dev/null
+++ b/src/WidgetValidationBase.php
@@ -0,0 +1,64 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity_browser\WidgetValidationBase.
+ */
+
+namespace Drupal\entity_browser;
+
+use Drupal\Core\Plugin\PluginBase;
+use Drupal\Core\TypedData\DataDefinitionInterface;
+
+/**
+ * Base implementation for widget validation plugins.
+ */
+abstract class WidgetValidationBase extends PluginBase implements WidgetValidationInterface {
+
+  /**
+   * Plugin label.
+   *
+   * @var string
+   */
+  protected $label;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function label() {
+    $this->label;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate(array $entities, $options = []) {}
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfiguration() {
+    return $this->configuration;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setConfiguration(array $configuration) {
+    $this->configuration = $configuration;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function calculateDependencies() {
+    return [];
+  }
+}
diff --git a/src/WidgetValidationInterface.php b/src/WidgetValidationInterface.php
new file mode 100644
index 0000000..3ae0653
--- /dev/null
+++ b/src/WidgetValidationInterface.php
@@ -0,0 +1,40 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity_browser\WidgetValidationInterface.
+ */
+
+namespace Drupal\entity_browser;
+
+use Drupal\Component\Plugin\ConfigurablePluginInterface;
+use Drupal\Component\Plugin\PluginInspectionInterface;
+
+
+/**
+ * Defines the interface for entity browser widget validations.
+ */
+interface WidgetValidationInterface extends PluginInspectionInterface, ConfigurablePluginInterface {
+
+  /**
+   * Returns the widget validation label.
+   *
+   * @return string
+   *   The widget validation label.
+   */
+  public function label();
+
+  /**
+   * Validates the widget.
+   *
+   * @param array $entities
+   *   Array of selected entities.
+   * @param array $options
+   *   (Optional) Array of options needed by the constraint validator.
+   *
+   * @return \Symfony\Component\Validator\ConstraintViolationListInterface
+   *   A list of constraint violations. If the list is empty, validation
+   *   succeeded.
+   */
+  public function validate(array $entities, $options = []);
+}
diff --git a/src/WidgetValidationManager.php b/src/WidgetValidationManager.php
new file mode 100644
index 0000000..787d67f
--- /dev/null
+++ b/src/WidgetValidationManager.php
@@ -0,0 +1,37 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\entity_browser\WidgetValidationManager.
+ */
+
+namespace Drupal\entity_browser;
+
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Extension\ModuleHandlerInterface;
+use Drupal\Core\Plugin\DefaultPluginManager;
+
+/**
+ * Manages entity browser widget validation plugins.
+ */
+class WidgetValidationManager extends DefaultPluginManager {
+
+  /**
+   * Constructs a new WidgetValidationManager.
+   *
+   * @param \Traversable $namespaces
+   *   An object that implements \Traversable which contains the root paths
+   *   keyed by the corresponding namespace to look for plugin implementations.
+   * @param \Drupal\Core\Cache\CacheBackendInterface $cache_backend
+   *   Cache backend instance to use.
+   * @param \Drupal\Core\Extension\ModuleHandlerInterface $module_handler
+   *   The module handler.
+   */
+  public function __construct(\Traversable $namespaces, CacheBackendInterface $cache_backend, ModuleHandlerInterface $module_handler) {
+    parent::__construct('Plugin/EntityBrowser/WidgetValidation', $namespaces, $module_handler, 'Drupal\entity_browser\WidgetValidationInterface', 'Drupal\entity_browser\Annotation\EntityBrowserWidgetValidation');
+
+    $this->alterInfo('entity_browser_widget_validation_info');
+    $this->setCacheBackend($cache_backend, 'entity_browser_widget_validation_plugins');
+  }
+
+}