diff --git a/core/lib/Drupal/Core/Field/WidgetBase.php b/core/lib/Drupal/Core/Field/WidgetBase.php
index b1579f2445..b399166df7 100644
--- a/core/lib/Drupal/Core/Field/WidgetBase.php
+++ b/core/lib/Drupal/Core/Field/WidgetBase.php
@@ -241,8 +241,21 @@ protected function formMultipleElements(FieldItemListInterface $items, array &$f
           ],
         ];
       }
-    }
 
+      // Allow modules to alter the field multiple widget form element.
+      $context = [
+        'form' => $form,
+        'widget' => $this,
+        'items' => $items,
+        'delta' => $delta,
+        'default' => $this->isDefaultValueWidget($form_state),
+        'cardinality' => $cardinality,
+      ];
+      \Drupal::moduleHandler()->alter([
+         'field_widget_multiple_form',
+         'field_widget_multiple_' . $this->getPluginId() . '_form',
+      ], $elements, $form_state, $context);
+    }
     return $elements;
   }
 
@@ -311,12 +324,14 @@ public static function addMoreAjax(array $form, FormStateInterface $form_state)
    * Generates the form element for a single copy of the widget.
    */
   protected function formSingleElement(FieldItemListInterface $items, $delta, array $element, array &$form, FormStateInterface $form_state) {
+      $cardinality = $this->fieldDefinition->getFieldStorageDefinition()->getCardinality();
     $element += [
       '#field_parents' => $form['#parents'],
       // Only the first widget should be required.
       '#required' => $delta == 0 && $this->fieldDefinition->isRequired(),
       '#delta' => $delta,
       '#weight' => $delta,
+      '#cardinality' => $cardinality,
     ];
 
     $element = $this->formElement($items, $delta, $element, $form, $form_state);
@@ -329,6 +344,7 @@ protected function formSingleElement(FieldItemListInterface $items, $delta, arra
         'items' => $items,
         'delta' => $delta,
         'default' => $this->isDefaultValueWidget($form_state),
+        'cardinality' => $cardinality,
       ];
       \Drupal::moduleHandler()->alter(['field_widget_form', 'field_widget_' . $this->getPluginId() . '_form'], $element, $form_state, $context);
     }
diff --git a/core/modules/field/field.api.php b/core/modules/field/field.api.php
index 130f923180..1fd8427e8c 100644
--- a/core/modules/field/field.api.php
+++ b/core/modules/field/field.api.php
@@ -183,6 +183,7 @@ function hook_field_widget_info_alter(array &$info) {
  * @see \Drupal\Core\Field\WidgetBaseInterface::form()
  * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
  * @see hook_field_widget_WIDGET_TYPE_form_alter()
+ * @see hook_field_widget_multiple_form_alter()
  */
 function hook_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
   // Add a css class to widget form elements for all fields of type mytype.
@@ -212,6 +213,7 @@ function hook_field_widget_form_alter(&$element, \Drupal\Core\Form\FormStateInte
  * @see \Drupal\Core\Field\WidgetBaseInterface::form()
  * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
  * @see hook_field_widget_form_alter()
+ * @see hook_field_widget_multiple_WIDGET_TYPE_form_alter()
  */
 function hook_field_widget_WIDGET_TYPE_form_alter(&$element, \Drupal\Core\Form\FormStateInterface $form_state, $context) {
   // Code here will only act on widgets of type WIDGET_TYPE.  For example,
@@ -220,6 +222,77 @@ function hook_field_widget_WIDGET_TYPE_form_alter(&$element, \Drupal\Core\Form\F
   $element['#autocomplete_route_name'] = 'mymodule.autocomplete_route';
 }
 
+/**
+ * Alter forms for multivalue field widgets provided by other modules.
+ *
+ * To adjust the individual widgets for the values themselves, use
+ * hook_field_widget_form_alter() instead.
+ *
+ * @param array $elements
+ *   The field widget form elements as constructed by
+ *   \Drupal\Core\Field\WidgetBaseInterface::formMultipleElements().
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ *   The current state of the form.
+ * @param array $context
+ *   An associative array containing the following key-value pairs:
+ *   - form: The form structure to which widgets are being attached. This may be
+ *     a full form structure, or a sub-element of a larger form.
+ *   - widget: The widget plugin instance.
+ *   - items: The field values, as a
+ *     \Drupal\Core\Field\FieldItemListInterface object.
+ *   - delta: The order of this item in the array of subelements (0, 1, 2, etc).
+ *   - default: A boolean indicating whether the form is being shown as a dummy
+ *     form to set default values.
+ *
+ * @see \Drupal\Core\Field\WidgetBaseInterface::form()
+ * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
+ * @see hook_field_widget_form_alter()
+ * @see hook_field_widget_multiple_WIDGET_TYPE_form_alter()
+ */
+function hook_field_widget_multiple_form_alter(array &$elements, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
+  // Add a css class to the overall element of any widget.
+  $elements['#attributes']['class'][] = 'myclass';
+}
+
+/**
+ * Alter multi-value widget forms for a widget provided by another module.
+ *
+ * Modules can implement hook_field_widget_multiple_WIDGET_TYPE_form_alter() to
+ * modify a specific widget form, rather than using
+ * hook_field_widget_form_alter() and checking the widget type.
+ *
+ * To modify the widget for individual items, use
+ * hook_field_widget_WIDGET_TYPE_form_alter() instead.
+ *
+ * @param array $elements
+ *   The field widget form elements as constructed by
+ *   \Drupal\Core\Field\WidgetBaseInterface::formMultipleElements().
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ *   The current state of the form.
+ * @param array $context
+ *   An associative array. See hook_field_widget_form_alter() for the structure
+ *   and content of the array.
+ *
+ * @see \Drupal\Core\Field\WidgetBaseInterface::form()
+ * @see \Drupal\Core\Field\WidgetBase::formSingleElement()
+ * @see hook_field_widget_multiple_form_alter()
+ */
+function hook_field_widget_multiple_WIDGET_TYPE_form_alter(array &$elements, \Drupal\Core\Form\FormStateInterface $form_state, array $context) {
+  // Alter the overall title for the multivalue widget (the table header,
+  // above the rows).
+  $elements['#title'] = t('Review your %title entries', ['%title' => $elements['#title']]);
+
+  if (!empty($elements['#description'])) {
+    // Add a paragraph to the description, above all the rows.
+    $elements['#description'] = [
+      '#markup' => $elements['#description'],
+    ];
+  }
+  else {
+    $elements['#description'] = t('These items might be out of date.');
+  }
+}
+
 /**
  * @} End of "defgroup field_widget".
  */
diff --git a/core/modules/media/media.module b/core/modules/media/media.module
index 296945973a..d3575457cc 100644
--- a/core/modules/media/media.module
+++ b/core/modules/media/media.module
@@ -9,6 +9,7 @@
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\Element;
+use Drupal\Core\Render\Markup;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Url;
@@ -66,6 +67,10 @@ function media_theme() {
     'media' => [
       'render element' => 'elements',
     ],
+    'media_reference_help' => [
+      'render element' => 'element',
+      'base hook' => 'field_multiple_value_form',
+    ],
   ];
 }
 
@@ -172,3 +177,54 @@ function media_form_field_ui_field_storage_add_form_alter(&$form, FormStateInter
   $form['add']['new_storage_type']['#weight'] = 0;
   $form['add']['description_wrapper']['#weight'] = 1;
 }
+
+/**
+ * Implements hook_field_widget_multiple_WIDGET_TYPE_form_alter().
+ */
+function media_field_widget_multiple_entity_reference_autocomplete_form_alter(array &$elements, FormStateInterface $form_state, array $context) {
+  // Add a description to inform users where they can find and add new media.
+  // Only add the description on autocomplete widgets for entity reference
+  // field pointing to media.
+  if (!empty($elements[0]['target_id']['#type']) && ($elements[0]['target_id']['#type'] === 'entity_autocomplete') && ($elements[0]['target_id']['#target_type'] === 'media')) {
+    $elements['#theme'] = 'media_reference_help';
+
+    // Customize the label for the field widget.
+    $label = $elements['#title'];
+    $elements['#original_label'] = $label;
+    $use_existing_label = t('Use existing %title', ['%title' => $label]);
+
+    if ($context['cardinality'] === 1) {
+      $elements[0]['target_id']['#title'] = $use_existing_label;
+    }
+    else {
+      $elements['#title'] = $use_existing_label;
+    }
+  }
+}
+
+/**
+ * Implements hook_preprocess_HOOK() for media reference widgets.
+ */
+function media_preprocess_media_reference_help(&$variables) {
+  $element = $variables['element'];
+  // Use the user-defined label for the field to indicate that the form allows
+  // reusing existing media.
+  $variables['label'] = $element['#original_label'];
+
+  if (!empty($element[0]['target_id']['#selection_settings']['target_bundles'])) {
+    $create_access = FALSE;
+    $bundles = $element[0]['target_id']['#selection_settings']['target_bundles'];
+    $variables['types'] = implode(", ", $bundles);
+    foreach ($bundles as $bundle) {
+      if (\Drupal::entityTypeManager()->getAccessControlHandler('media')->createAccess($bundle, \Drupal::currentUser())) {
+        $create_access = TRUE;
+      }
+    }
+    if ($create_access === TRUE) {
+      $variables['media_add_page'] = Url::fromRoute('entity.media.add_page')->toString();
+    }
+    if (\Drupal::currentUser()->hasPermission('access media overview')) {
+      $variables['media_list'] = Url::fromRoute('entity.media.collection')->toString();
+    }
+  }
+}
diff --git a/core/modules/media/templates/media-reference-help.html.twig b/core/modules/media/templates/media-reference-help.html.twig
new file mode 100644
index 0000000000..1b23f5f234
--- /dev/null
+++ b/core/modules/media/templates/media-reference-help.html.twig
@@ -0,0 +1,60 @@
+{#
+/**
+ * @file
+ * Theme override for media reference fields.
+ *
+ * @todo I have no idea what I'm doing.
+ *
+ * @see template_preprocess_field_multiple_value_form()
+ */
+#}
+<fieldset>
+  <legend>{{ label }}</legend>
+
+  {% if media_add_page %}
+    <h4>
+      {% trans %}
+        Create new {{ label|placeholder }}
+      {% endtrans %}
+    </h4>
+    {% trans %}
+      Create your media on the <a href="{{ media_add_page }}" target="_blank">media add page</a> (opens a new window), then add it by name to the field below.
+    {% endtrans %}
+  {% endif %}
+
+  {% if multiple %}
+    <div class="js-form-item form-item">
+      {{ table }}
+  {% else %}
+    {% for element in elements %}
+      {{ element }}
+    {% endfor %}
+  {% endif %}
+
+  <div{{ description.attributes.addClass('description') }} >
+    {% trans %}
+      Type part of the media name.
+    {% endtrans %}
+    {% if media_list %}
+      {% trans %}
+        See the <a href="{{ media_list }}" target="_blank">media list</a> (opens a new window) to help locate media.
+      {% endtrans %}
+    {% endif %}
+    {% trans %}
+      Allowed media types: {{ types|placeholder }}
+    {% endtrans %}
+    {% if multiple %}
+      {% if description.content %}
+        {{ description.content }}
+      {% endif %}
+      {% if button %}
+        <div class="clearfix">{{ button }}</div>
+      {% endif %}
+    {% endif %}
+
+  </div>
+
+  {% if multiple %}
+    </div>
+  {% endif %}
+</fieldset>
diff --git a/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php b/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php
index 224bb426e3..379ba74717 100644
--- a/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php
+++ b/core/modules/media/tests/src/Functional/MediaFunctionalTestTrait.php
@@ -14,6 +14,7 @@
    */
   protected static $adminUserPermissions = [
     // Media module permissions.
+    'access media overview',
     'administer media',
     'administer media fields',
     'administer media form display',
diff --git a/core/modules/media/tests/src/Functional/MediaUiFunctionalTest.php b/core/modules/media/tests/src/Functional/MediaUiFunctionalTest.php
index 8d876b7b0d..3497f41c73 100644
--- a/core/modules/media/tests/src/Functional/MediaUiFunctionalTest.php
+++ b/core/modules/media/tests/src/Functional/MediaUiFunctionalTest.php
@@ -3,6 +3,10 @@
 namespace Drupal\Tests\media\Functional;
 
 use Drupal\media\Entity\Media;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Url;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
 
 /**
  * Ensures that media UI works correctly.
@@ -192,6 +196,184 @@ public function testRenderedEntityReferencedMedia() {
     $assert_session->fieldValueEquals('fields[field_foo_field][type]', 'entity_reference_entity_view');
   }
 
+  public function providerTestMediaReferenceWidget() {
+    return [
+      // Single-value fields with a single media type:
+      // - The user can create and list the media.
+      [1, [TRUE], TRUE],
+
+      // - The user can list but not create the media.
+      [1, [FALSE], TRUE],
+      // - The user can create but not list the media.
+      [1, [TRUE], FALSE],
+      // - The user can neither create nor list the media.
+      [1, [FALSE], FALSE],
+
+      // Single-value fields with two media types:
+      // - The user can create both types.
+      [1, [TRUE, TRUE], TRUE],
+      // - The user can create only one type.
+      [1, [TRUE, FALSE], TRUE],
+      // - The user cannot create either type.
+      [1, [FALSE, FALSE], TRUE],
+
+      // Multiple-value field with a cardinality of 3, with media the user can
+      // create and list.
+      [3, [TRUE], TRUE],
+
+      // Unlimited value field.
+      [FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED, [TRUE], TRUE],
+
+    ];
+  }
+
+  /**
+   * Test the default autocomplete widgets for media reference fields.
+   *
+   * @dataProvider providerTestMediaReferenceWidget
+   *
+   * @see media_field_widget_entity_reference_autocomplete_form_alter()
+   * @see media_field_widget_multiple_entity_reference_autocomplete_form_alter()
+   */
+  public function testMediaReferenceWidget($cardinality, array $media_type_create_access, $list_access) {
+    $assert_session = $this->assertSession();
+
+    // Create a non-media entity reference and make sure our help text is not
+    // showing up.
+    $non_media_content_type = $this->createContentType();
+    $non_media_storage = FieldStorageConfig::create([
+      'field_name' => 'field_not_a_media_field',
+      'entity_type' => 'node',
+      'type' => 'entity_reference',
+      'cardinality' => FieldStorageDefinitionInterface::CARDINALITY_UNLIMITED,
+      'settings' => [
+        'target_type' => 'node',
+      ],
+    ]);
+    $non_media_storage->save();
+    $non_media_field = FieldConfig::create([
+      'label' => 'No media here!',
+      'field_storage' => $non_media_storage,
+      'entity_type' => 'node',
+      'bundle' => $non_media_content_type->id(),
+      'settings' => [
+        'handler' => 'default',
+        'handler_settings' => [
+          'target_bundles' => [
+            $non_media_content_type->id() => $non_media_content_type->id(),
+          ],
+        ],
+      ],
+    ]);
+    $non_media_field->save();
+    entity_get_form_display('node', $non_media_content_type->id(), 'default')
+      ->setComponent('field_not_a_media_field', [
+        'type' => 'entity_reference_autocomplete',
+      ])
+      ->save();
+    $this->drupalGet("/node/add/{$non_media_content_type->id()}");
+    $assert_session->pageTextNotContains('Create your media on the media add page (opens a new window), then add it by name to the field below.');
+    $assert_session->pageTextNotContains('Type part of the media name. See the media list (opens a new window) to help locate media.');
+
+    // Now create a separate content type with the media field.
+    $content_type = $this->createContentType();
+
+    // Create media types to use in the field.
+    $media_types = [];
+    $permissions = [];
+    $any_create_access = FALSE;
+    foreach ($media_type_create_access as $id => $access) {
+      if ($access) {
+        $any_create_access = TRUE;
+        $permissions[] = "create media_type_$id media";
+      }
+      $media_type = $this->createMediaType(['bundle' => "media_type_$id"]);
+      $media_types["media_type_$id"] = "media_type_$id";
+    }
+    $type_list = implode(", ", array_keys($media_types));
+
+    // Create a user that can create content of the type, with other
+    // permissions as given by the data provider.
+    $permissions[] = "create {$content_type->id()} content";
+    if ($list_access) {
+      $permissions[] = "access media overview";
+    }
+    $test_user = $this->drupalCreateUser($permissions);
+
+    // Create the field.
+    $storage = FieldStorageConfig::create([
+      'field_name' => "field_media_reference",
+      'entity_type' => 'node',
+      'type' => 'entity_reference',
+      'cardinality' => $cardinality,
+      'settings' => [
+        'target_type' => 'media',
+      ],
+    ]);
+    $storage->save();
+    $field = FieldConfig::create([
+      'label' => "Media (cardinality $cardinality)",
+      'field_storage' => $storage,
+      'entity_type' => 'node',
+      'bundle' => $content_type->id(),
+      'settings' => [
+        'handler' => 'default',
+        'handler_settings' => [
+          'target_bundles' => $media_types,
+        ],
+      ],
+    ]);
+    $field->save();
+    entity_get_form_display('node', $content_type->id(), 'default')
+      ->setComponent('field_media_reference', [
+        'type' => 'entity_reference_autocomplete',
+      ])
+      ->save();
+
+    // Now, xheck that the widget displays the expected help text under the
+    // given conditions for the test user.
+    $this->drupalLogin($test_user);
+    $this->drupalGet("/node/add/{$content_type->id()}");
+    $fieldset_selector = '#edit-field-media-reference-wrapper fieldset';
+    $fieldset = $assert_session->elementExists('css', $fieldset_selector);
+
+    $this->assertEquals($field->getLabel(), $fieldset->find('css', 'legend')->getText());
+
+    // Find all the links inside the fieldset.
+    $find_all_links = $fieldset->findAll('css', 'a');
+    $links = [];
+    foreach ($find_all_links as $link) {
+      $links[$link->getText()] = $link;
+    }
+
+    // Within the fieldset, only once for each.
+    if ($any_create_access) {
+      $assert_session->elementsCount('css', $fieldset_selector . ":contains('Create new {$field->getLabel()}')", 1);
+      $assert_session->elementsCount('css', $fieldset_selector . ":contains('Create your media on the media add page (opens a new window), then add it by name to the field below.')", 1);
+      $this->assertTrue(in_array('media add page', array_keys($links)));
+      $this->assertEquals(Url::fromRoute('entity.media.add_page')->toString(), $links['media add page']->getAttribute('href'));
+      $this->assertEquals('_blank', $links['media add page']->getAttribute('target'));
+    }
+    else {
+      $assert_session->pageTextNotContains("Create new {$field->getLabel()}");
+      $assert_session->pageTextNotContains("Create your media on the media add page (opens a new window), then add it by name to the field below.");
+      $this->assertFalse(in_array('media add page', array_keys($links)));
+    }
+    $assert_session->elementTextContains('css', $fieldset_selector, "Use existing {$field->getLabel()}");
+    $assert_session->elementTextContains('css', $fieldset_selector, "Type part of the media name.");
+    if ($list_access) {
+      $assert_session->elementTextContains('css', $fieldset_selector, "See the media list (opens a new window) to help locate media.");
+      $this->assertTrue(in_array('media list', array_keys($links)));
+      $this->assertEquals(Url::fromRoute('entity.media.collection')->toString(), $links['media list']->getAttribute('href'));
+      $this->assertEquals('_blank', $links['media list']->getAttribute('target'));
+    }
+    else {
+      $assert_session->pageTextNotContains("See the media list (opens a new window) to help locate media.");
+      $this->assertFalse(in_array('media list', array_keys($links)));
+    }
+    $assert_session->elementTextContains('css', $fieldset_selector, "Allowed media types: $type_list");
+  }
+
   /**
    * Test the media collection route.
    */
diff --git a/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php
index 7d22484238..46f7858df6 100644
--- a/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaReferenceFieldHelpTest.php
@@ -3,6 +3,10 @@
 namespace Drupal\Tests\media\FunctionalJavascript;
 
 use Drupal\Component\Utility\Html;
+use Drupal\Core\Field\FieldStorageDefinitionInterface;
+use Drupal\Core\Url;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
 
 /**
  * Tests related to media reference fields.
