diff --git a/core/lib/Drupal/Core/Entity/ContentEntityBase.php b/core/lib/Drupal/Core/Entity/ContentEntityBase.php
index 846e89a..fad421d 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityBase.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityBase.php
@@ -156,6 +156,13 @@
   protected $loadedRevisionId;
 
   /**
+   * Whether to serialize the complete entity structure.
+   *
+   * @var bool
+   */
+  protected $deepSerialization = FALSE;
+
+  /**
    * {@inheritdoc}
    */
   public function __construct(array $values, $entity_type, $bundle = FALSE, $translations = array()) {
@@ -440,7 +447,7 @@ public function __sleep() {
     // Get the values of instantiated field objects, only serialize the values.
     foreach ($this->fields as $name => $fields) {
       foreach ($fields as $langcode => $field) {
-        $this->values[$name][$langcode] = $field->getValue();
+        $this->values[$name][$langcode] = $field->getSerializationValue($this->deepSerialization);
       }
     }
     $this->fields = array();
@@ -448,12 +455,36 @@ public function __sleep() {
     $this->languages = NULL;
     $this->clearTranslationCache();
 
+    // As during the serialization process referenced entities might also have
+    // the "deepSerialization" flag set the only place of unsetting it remains
+    // in the sleep method right after the entity is prepared for the
+    // serialization and before executing it.
+    // @see \Drupal\Core\Field\EntityReferenceFieldItemList::getSerializationValue().
+    $this->deepSerialization = FALSE;
+
     return parent::__sleep();
   }
 
   /**
    * {@inheritdoc}
    */
+  public function deepSerialize() {
+    // When running a deep serialization the flag "deepSerialization" has to be
+    // set in order to serialize referenced entities as well.
+    $this->deepSerialization = TRUE;
+    return serialize($this);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function setDeepSerialization() {
+    $this->deepSerialization = TRUE;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function id() {
     return $this->getEntityKey('id');
   }
@@ -823,6 +854,7 @@ protected function initializeTranslation($langcode) {
     $translation->enforceIsNew = &$this->enforceIsNew;
     $translation->newRevision = &$this->newRevision;
     $translation->entityKeys = &$this->entityKeys;
+    $translation->deepSerialization = &$this->deepSerialization;
     $translation->translatableEntityKeys = &$this->translatableEntityKeys;
     $translation->translationInitialize = FALSE;
     $translation->typedData = NULL;
@@ -1099,7 +1131,8 @@ public function __clone() {
 
       // Ensure that the following properties are actually cloned by
       // overwriting the original references with ones pointing to copies of
-      // them: enforceIsNew, newRevision and loadedRevisionId.
+      // them: enforceIsNew, newRevision, loadedRevisionId and
+      // deepSerialization.
       $enforce_is_new = $this->enforceIsNew;
       $this->enforceIsNew = &$enforce_is_new;
 
@@ -1108,6 +1141,9 @@ public function __clone() {
 
       $original_revision_id = $this->loadedRevisionId;
       $this->loadedRevisionId = &$original_revision_id;
+
+      $deepSerialization = $this->deepSerialization;
+      $this->deepSerialization = &$deepSerialization;
     }
   }
 
diff --git a/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php b/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php
index d21a2a8..3f98226 100644
--- a/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php
+++ b/core/lib/Drupal/Core/Entity/FieldableEntityInterface.php
@@ -236,4 +236,23 @@ public function isValidationRequired();
    */
   public function setValidationRequired($required);
 
+  /**
+   * Returns a deeply serialized entity.
+   *
+   * @return string
+   *   The deeply serialized entity.
+   */
+  public function deepSerialize();
+
+  /**
+   * Flags the entity for deep serialization.
+   *
+   * After calling this method and serializing the entity e.g.
+   * "serialize($entity);" a deep serialization will be performed and the flag
+   * will be automatically reset.
+   *
+   * @return $this
+   */
+  public function setDeepSerialization();
+
 }
diff --git a/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
index d21ac96..b471d4d 100644
--- a/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
+++ b/core/lib/Drupal/Core/Field/EntityReferenceFieldItemList.php
@@ -13,6 +13,25 @@ class EntityReferenceFieldItemList extends FieldItemList implements EntityRefere
   /**
    * {@inheritdoc}
    */
+  public function getSerializationValue($deep_serialization) {
+    if ($deep_serialization) {
+      $values = $this->getValue(TRUE);
+      foreach ($values as $delta => $item_values) {
+        if (isset($values[$delta]['entity']) && ($values[$delta]['entity'] instanceof FieldableEntityInterface)) {
+          $values[$delta]['entity']->setDeepSerialization();
+        }
+      }
+    }
+    else {
+      $values = parent::getSerializationValue($deep_serialization);
+    }
+
+    return $values;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function getConstraints() {
     $constraints = parent::getConstraints();
     $constraint_manager = $this->getTypedDataManager()->getValidationConstraintManager();
diff --git a/core/lib/Drupal/Core/Field/FieldItemList.php b/core/lib/Drupal/Core/Field/FieldItemList.php
index 97dd493..ea530d1 100644
--- a/core/lib/Drupal/Core/Field/FieldItemList.php
+++ b/core/lib/Drupal/Core/Field/FieldItemList.php
@@ -121,6 +121,13 @@ public function setValue($values, $notify = TRUE) {
   /**
    * {@inheritdoc}
    */
+  public function getSerializationValue($deep_serialization) {
+    return $this->getValue();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function __get($property_name) {
     // For empty fields, $entity->field->property is NULL.
     if ($item = $this->first()) {
diff --git a/core/lib/Drupal/Core/Field/FieldItemListInterface.php b/core/lib/Drupal/Core/Field/FieldItemListInterface.php
index 4e77202..dfeeede 100644
--- a/core/lib/Drupal/Core/Field/FieldItemListInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldItemListInterface.php
@@ -271,4 +271,14 @@ public static function processDefaultValue($default_value, FieldableEntityInterf
    */
   public function equals(FieldItemListInterface $list_to_compare);
 
+  /**
+   * Gets the data value prepared for serialization.
+   *
+   * @param bool $deep_serialization
+   *   Whether a deep serialization is running.
+   *
+   * @return mixed
+   */
+  public function getSerializationValue($deep_serialization);
+
 }
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
index 3babf42..8907869 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldType/EntityReferenceItem.php
@@ -190,7 +190,7 @@ public function setValue($values, $notify = TRUE) {
         // If the entity has been saved and we're trying to set both the
         // target_id and the entity values with a non-null target ID, then the
         // value for target_id should match the ID of the entity value.
-        if (!$this->entity->isNew() && $values['target_id'] !== NULL && ($entity_id !== $values['target_id'])) {
+        if (!$this->entity->isNew() && $values['target_id'] !== NULL && ($entity_id != $values['target_id'])) {
           throw new \InvalidArgumentException('The target id and entity passed to the entity reference item do not match.');
         }
       }
@@ -205,12 +205,12 @@ public function setValue($values, $notify = TRUE) {
   /**
    * {@inheritdoc}
    */
-  public function getValue() {
+  public function getValue($include_computed = FALSE) {
     $values = parent::getValue();
 
     // If there is an unsaved entity, return it as part of the field item values
     // to ensure idempotency of getValue() / setValue().
-    if ($this->hasNewEntity()) {
+    if (($include_computed && !$this->isEmpty()) || $this->hasNewEntity()) {
       $values['entity'] = $this->entity;
     }
     return $values;
diff --git a/core/tests/Drupal/KernelTests/Core/Entity/ContentEntitySerializationTest.php b/core/tests/Drupal/KernelTests/Core/Entity/ContentEntitySerializationTest.php
new file mode 100644
index 0000000..7e2a442
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Entity/ContentEntitySerializationTest.php
@@ -0,0 +1,121 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Entity;
+
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
+
+/**
+ * Tests the serialization of content entities.
+ *
+ * @group Entity
+ */
+class ContentEntitySerializationTest extends EntityKernelTestBase {
+
+  /**
+   * Modules to enable.
+   *
+   * @var array
+   */
+  public static $modules = ['user', 'system', 'field', 'entity_test'];
+
+  /**
+   * The entity type id.
+   *
+   * @var string
+   */
+  protected $entityTypeId = 'entity_test_mul';
+
+  /**
+   * The EntityTestMul entity type storage.
+   *
+   * @var \Drupal\Core\Entity\ContentEntityStorageInterface
+   */
+  protected $entityStorage;
+
+  /**
+   * The entity reference field name.
+   *
+   * @var string
+   */
+  protected $entityReferenceFieldName = 'field_test_entity_reference';
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->installEntitySchema($this->entityTypeId);
+    $this->entityStorage = $this->entityManager->getStorage($this->entityTypeId);
+
+    // Create the test entity reference field.
+    FieldStorageConfig::create([
+      'field_name' => $this->entityReferenceFieldName,
+      'entity_type' => $this->entityTypeId,
+      'type' => 'entity_reference',
+      'settings' => [
+        'target_type' => $this->entityTypeId,
+      ],
+    ])->save();
+    FieldConfig::create([
+      'entity_type' => $this->entityTypeId,
+      'field_name' => $this->entityReferenceFieldName,
+      'bundle' => $this->entityTypeId,
+      'translatable' => TRUE,
+    ])->save();
+  }
+
+  /**
+   * Tests regular and deep serialization.
+   */
+  public function testSerialization() {
+    $this->doTestSerialization(FALSE);
+    $this->doTestSerialization(TRUE);
+  }
+
+  /**
+   * Tests regular or deep serialization based on the parameter.
+   *
+   * @param bool $deep_serialization
+   *   Whether to test regular or deep serialization.
+   */
+  protected function doTestSerialization($deep_serialization) {
+    $field_name = $this->entityReferenceFieldName;
+    $user = $this->createUser();
+    $initial_entity_name = 'test test';
+
+    // Create the test entities.
+    /** @var \Drupal\Core\Entity\ContentEntityInterface $entity_level_zero */
+    $entity_level_zero = $this->entityStorage->create([
+      'name' => $initial_entity_name,
+      'user_id' => $user->id(),
+      'language' => 'en',
+    ]);
+    $entity_level_zero->save();
+    $entity_level_one = $entity_level_zero->createDuplicate();
+    $entity_level_one->save();
+    $entity_level_two = $entity_level_zero->createDuplicate();
+    $entity_level_two->save();
+
+    $entity_level_zero->$field_name->appendItem($entity_level_one);
+    $entity_level_zero->save();
+    $entity_level_one->$field_name->appendItem($entity_level_two);
+    $entity_level_one->save();
+
+    // Change the entity values in memory but do not persist them into the
+    // storage.
+    $entity_level_zero->name = 'entity level zero';
+    $entity_level_zero->$field_name->entity->name = 'entity level one';
+    $entity_level_zero->$field_name->entity->$field_name->entity->name = 'entity level two';
+
+    $serialized_entity = $deep_serialization ? $entity_level_zero->deepSerialize() : serialize($entity_level_zero);
+    $entity_level_zero = unserialize($serialized_entity);
+
+    // If regular serialization is tested then the changes on the referenced
+    // entities should be lost, but kept if deep serialization is tested.
+    $this->assertEquals('entity level zero', $entity_level_zero->label());
+    $this->assertEquals($deep_serialization ? 'entity level one' : $initial_entity_name, $entity_level_zero->$field_name->entity->label());
+    $this->assertEquals($deep_serialization ? 'entity level two' : $initial_entity_name, $entity_level_zero->$field_name->entity->$field_name->entity->label());
+  }
+
+}