diff --git a/modules/comment/comment.module b/modules/comment/comment.module
index 429c3b0..9820568 100644
--- a/modules/comment/comment.module
+++ b/modules/comment/comment.module
@@ -992,6 +992,14 @@ function comment_build_content($comment, $node, $view_mode = 'full', $langcode =
   // Remove previously built content, if exists.
   $comment->content = array();
 
+  // Allow modules to change the view mode.
+  $context = array(
+    'entity_type' => 'comment',
+    'entity' => $comment,
+    'langcode' => $langcode,
+  );
+  drupal_alter('entity_view_mode', $view_mode, $context);
+
   // Build fields content.
   field_attach_prepare_view('comment', array($comment->cid => $comment), $view_mode, $langcode);
   entity_prepare_view('comment', array($comment->cid => $comment), $langcode);
diff --git a/modules/node/node.module b/modules/node/node.module
index 2baee43..e3e3f9e 100644
--- a/modules/node/node.module
+++ b/modules/node/node.module
@@ -1345,6 +1345,14 @@ function node_build_content($node, $view_mode = 'full', $langcode = NULL) {
   // Remove previously built content, if exists.
   $node->content = array();
 
+  // Allow modules to change the view mode.
+  $context = array(
+    'entity_type' => 'node',
+    'entity' => $node,
+    'langcode' => $langcode,
+  );
+  drupal_alter('entity_view_mode', $view_mode, $context);
+
   // The 'view' hook can be implemented to overwrite the default function
   // to display nodes.
   if (node_hook($node, 'view')) {
diff --git a/modules/node/node.test b/modules/node/node.test
index 7080ce7..3c9fbaa 100644
--- a/modules/node/node.test
+++ b/modules/node/node.test
@@ -2597,3 +2597,48 @@ class NodeAccessFieldTestCase extends NodeWebTestCase {
     $this->assertRaw($default, 'The updated default value is displayed when creating a new node.');
   }
 }
+
+/**
+ * Tests changing view modes for nodes.
+ */
+class NodeEntityViewModeAlterTest extends NodeWebTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Node entity view mode',
+      'description' => 'Test changing view mode.',
+      'group' => 'Node'
+    );
+  }
+
+  function setUp() {
+    parent::setUp(array('node_test'));
+  }
+
+  /**
+   * Create a "Basic page" node and verify its consistency in the database.
+   */
+  function testNodeViewModeChange() {
+    $web_user = $this->drupalCreateUser(array('create page content', 'edit own page content'));
+    $this->drupalLogin($web_user);
+
+    // Create a node.
+    $edit = array();
+    $langcode = LANGUAGE_NONE;
+    $edit["title"] = $this->randomName(8);
+    $edit["body[$langcode][0][value]"] = t('Data that should appear only in the body for the node.');
+    $edit["body[$langcode][0][summary]"] = t('Extra data that should appear only in the teaser for the node.');
+    $this->drupalPost('node/add/page', $edit, t('Save'));
+
+    $node = $this->drupalGetNodeByTitle($edit["title"]);
+
+    // Set the flag to alter the view mode and view the node.
+    variable_set('node_test_change_view_mode', 'teaser');
+    $this->drupalGet('node/' . $node->nid);
+
+    // Check that teaser mode is viewed.
+    $this->assertText('Extra data that should appear only in the teaser for the node.', 'Teaser text present');
+    // Make sure body text is not present.
+    $this->assertNoText('Data that should appear only in the body for the node.', 'Body text not present');
+  }
+}
diff --git a/modules/node/tests/node_test.module b/modules/node/tests/node_test.module
index b0ebc14..a52c1fa 100644
--- a/modules/node/tests/node_test.module
+++ b/modules/node/tests/node_test.module
@@ -149,3 +149,13 @@ function node_test_node_update($node) {
     }
   }
 }
+
+/**
+ * Implements hook_entity_view_mode_alter().
+ */
+function node_test_entity_view_mode_alter(&$view_mode, $context) {
+  // Only alter the view mode if we are on the test callback.
+  if ($change_view_mode = variable_get('node_test_change_view_mode', '')) {
+    $view_mode = $change_view_mode;
+  }
+}
diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index fbb0a55..c51fc44 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -466,6 +466,24 @@ function hook_entity_view_alter(&$build, $type) {
 }
 
 /**
+ * Change the view mode of an entity that is being displayed.
+ *
+ * @param string $view_mode
+ *   The view_mode that is to be used to display the entity.
+ * @param array $context
+ *   Array with contextual information, including:
+ *   - entity_type: The type of the entity that is being viewed.
+ *   - entity: The entity object.
+ *   - langcode: The langcode the entity is being viewed in.
+ */
+function hook_entity_view_mode_alter(&$view_mode, $context) {
+  // For nodes, change the view mode when it is teaser.
+  if ($context['entity_type'] == 'node' && $view_mode == 'teaser') {
+    $view_mode = 'my_custom_view_mode';
+  }
+}
+
+/**
  * Define administrative paths.
  *
  * Modules may specify whether or not the paths they define in hook_menu() are
@@ -1041,7 +1059,7 @@ function hook_menu_get_item_alter(&$router_item, $path, $original_map) {
  * This 'abc' object will then be passed into the callback functions defined
  * for the menu item, such as the page callback function mymodule_abc_edit()
  * to replace the integer 1 in the argument array. Note that a load function
- * should return FALSE when it is unable to provide a loadable object. For 
+ * should return FALSE when it is unable to provide a loadable object. For
  * example, the node_load() function for the 'node/%node/edit' menu item will
  * return FALSE for the path 'node/999/edit' if a node with a node ID of 999
  * does not exist. The menu routing system will return a 404 error in this case.
diff --git a/modules/taxonomy/taxonomy.module b/modules/taxonomy/taxonomy.module
index df4084c..c86c510 100644
--- a/modules/taxonomy/taxonomy.module
+++ b/modules/taxonomy/taxonomy.module
@@ -754,6 +754,14 @@ function taxonomy_term_view($term, $view_mode = 'full', $langcode = NULL) {
     $langcode = $GLOBALS['language_content']->language;
   }
 
+  // Allow modules to change the view mode.
+  $context = array(
+    'entity_type' => 'taxonomy_term',
+    'entity' => $term,
+    'langcode' => $langcode,
+  );
+  drupal_alter('entity_view_mode', $view_mode, $context);
+
   field_attach_prepare_view('taxonomy_term', array($term->tid => $term), $view_mode, $langcode);
   entity_prepare_view('taxonomy_term', array($term->tid => $term), $langcode);
 
diff --git a/modules/user/user.module b/modules/user/user.module
index 47ac642..bf3f3b1 100644
--- a/modules/user/user.module
+++ b/modules/user/user.module
@@ -2574,6 +2574,14 @@ function user_build_content($account, $view_mode = 'full', $langcode = NULL) {
   // Remove previously built content, if exists.
   $account->content = array();
 
+  // Allow modules to change the view mode.
+  $context = array(
+    'entity_type' => 'user',
+    'entity' => $account,
+    'langcode' => $langcode,
+  );
+  drupal_alter('entity_view_mode', $view_mode, $context);
+
   // Build fields content.
   field_attach_prepare_view('user', array($account->uid => $account), $view_mode, $langcode);
   entity_prepare_view('user', array($account->uid => $account), $langcode);