diff --git a/core/lib/Drupal/Core/Pager/PagerManager.php b/core/lib/Drupal/Core/Pager/PagerManager.php
index b3efb3143f..92014e93ec 100644
--- a/core/lib/Drupal/Core/Pager/PagerManager.php
+++ b/core/lib/Drupal/Core/Pager/PagerManager.php
@@ -10,23 +10,20 @@
  *
  * This class does not deal with pager render elements.
  *
- * Pager information can be sent through an HTTP request using the 'pager' query
- * key. The value for this key tells us the current page for an arbitrary number
- * of pagers.
+ * Since there can be multiple pagers per requested page, each one is
+ * represented by an 'element' ID. It represents the index of the pager element
+ * within the 'page' query. The pager element is an integer telling us the
+ * current page number for that pager. (Page numbering is zero-based.)
  *
- * Since there can be multiple pagers, each one has an 'element' ID. This is an
- * integer and is the location of the current page info within the 'pager'
- * query.
- *
- * This class generally replaces the pager.inc functions. Those functions use
- * globals to store data which they all use. Since we require backwards
- * compatibility with this behavior, this class presents a public API for using
- * pager information, which is implemented using the same globals as a
+ * This class generally replaces the functions in core/includes/pager.inc. Those
+ * functions use globals to store data which they all use. Since we require
+ * backwards compatibility with this behavior, this class presents a public API
+ * for using pager information, which is implemented using the same globals as a
  * 'backend.'
  *
  * @todo Re-implement the parts of this class that use globals in Drupal 9.
  */
-class PagerManager {
+class PagerManager implements PagerManagerInterface {
 
   /**
    * The HTTP request stack.
@@ -53,94 +50,7 @@ public function __construct(RequestStack $stack) {
   }
 
   /**
-   * Initializes a pager.
-   *
-   * This function sets up the necessary global variables so that the render
-   * system will correctly process #type 'pager' render arrays to output pagers
-   * that correspond to the items being displayed.
-   *
-   * If the items being displayed result from a database query performed using
-   * Drupal's database API, and if you have control over the construction of the
-   * database query, you do not need to call this function directly; instead, you
-   * can simply extend the query object with the 'PagerSelectExtender' extender
-   * before executing it. For example:
-   * @code
-   *   $query = db_select('some_table')
-   *     ->extend('Drupal\Core\Database\Query\PagerSelectExtender');
-   * @endcode
-   *
-   * However, if you are using a different method for generating the items to be
-   * paged through, then you should call this function in preparation.
-   *
-   * The following example shows how this function can be used in a controller
-   * that invokes an external datastore with an SQL-like syntax:
-   * @code
-   *   // First find the total number of items and initialize the pager.
-   *   $where = "status = 1";
-   *   $total = mymodule_select("SELECT COUNT(*) FROM data " . $where)->result();
-   *   $num_per_page = \Drupal::config('mymodule.settings')->get('num_per_page');
-   *   $page = pager_default_initialize($total, $num_per_page);
-   *
-   *   // Next, retrieve the items for the current page and put them into a
-   *   // render array.
-   *   $offset = $num_per_page * $page;
-   *   $result = mymodule_select("SELECT * FROM data " . $where . " LIMIT %d, %d", $offset, $num_per_page)->fetchAll();
-   *   $render = [];
-   *   $render[] = [
-   *     '#theme' => 'mymodule_results',
-   *     '#result' => $result,
-   *   ];
-   *
-   *   // Finally, add the pager to the render array, and return.
-   *   $render[] = ['#type' => 'pager'];
-   *   return $render;
-   * @endcode
-   *
-   * A second example involves a controller that invokes an external search
-   * service where the total number of matching results is provided as part of
-   * the returned set (so that we do not need a separate query in order to obtain
-   * this information). Here, we call pager_find_page() to calculate the desired
-   * offset before the search is invoked:
-   * @code
-   *   // Perform the query, using the requested offset from pager_find_page().
-   *   // This comes from a URL parameter, so here we are assuming that the URL
-   *   // parameter corresponds to an actual page of results that will exist
-   *   // within the set.
-   *   $page = pager_find_page();
-   *   $num_per_page = \Drupal::config('mymodule.settings')->get('num_per_page');
-   *   $offset = $num_per_page * $page;
-   *   $result = mymodule_remote_search($keywords, $offset, $num_per_page);
-   *
-   *   // Now that we have the total number of results, initialize the pager.
-   *   pager_default_initialize($result->total, $num_per_page);
-   *
-   *   // Create a render array with the search results.
-   *   $render = [];
-   *   $render[] = [
-   *     '#theme' => 'search_results',
-   *     '#results' => $result->data,
-   *     '#type' => 'remote',
-   *   ];
-   *
-   *   // Finally, add the pager to the render array, and return.
-   *   $render[] = ['#type' => 'pager'];
-   *   return $render;
-   * @endcode
-   *
-   * @param int $total
-   *   The total number of items to be paged.
-   * @param int $limit
-   *   The number of items the calling code will display per page.
-   * @param int $element
-   *   (optional) An integer to distinguish between multiple pagers on one page.
-   *
-   * @return int
-   *   The number of the current page, within the pager represented by $element.
-   *   This is determined from the URL query parameter
-   *   \Drupal::request()->query->get('page), or 0 by default. However, if a page
-   *   that does not correspond to the actual range of the result set was
-   *   requested, this function will return the closest page actually within the
-   *   result set.
+   * {@inheritdoc}
    */
   public function defaultInitialize($total, $limit, $element = 0) {
     $page = $this->findPage($element);
@@ -154,20 +64,7 @@ public function defaultInitialize($total, $limit, $element = 0) {
   }
 
   /**
-   * Returns the current page being requested for display within a pager.
-   *
-   * @param int $element
-   *   (optional) An integer to distinguish between multiple pagers on one page.
-   *
-   * @return int
-   *   The number of the current requested page, within the pager represented by
-   *   $element. This is determined from the URL query parameter
-   *   \Drupal::request()->query->get('page'), or 0 by default. Note that this
-   *   number may differ from the actual page being displayed. For example, if a
-   *   search for "example text" brings up three pages of results, but a user
-   *   visits search/node/example+text?page=10, this function will return 10,
-   *   even though the default pager implementation adjusts for this and still
-   *   displays the third page of search results at that URL.
+   * {@inheritdoc}
    */
   public function findPage($element = 0) {
     $page = $this->requestStack->getCurrentRequest()->query->get('page', '');
@@ -179,64 +76,45 @@ public function findPage($element = 0) {
   }
 
   /**
-   * Compose a URL query parameter array for pager links.
-   *
-   * @return array
-   *   A URL query parameter array that consists of all components of the current
-   *   page request except for those pertaining to paging.
+   * {@inheritdoc}
    */
   public function getQueryParameters() {
     if (empty($this->queryParameters)) {
-      $this->queryParameters = UrlHelper::filterQueryParameters($this->requestStack->getCurrentRequest()->query->all(), ['page']);
+      $this->queryParameters = UrlHelper::filterQueryParameters(
+        $this->requestStack->getCurrentRequest()->query->all(), ['page']
+      );
     }
     return $this->queryParameters;
   }
 
   /**
-   * Gets the URL query parameter array of a pager link.
-   *
-   * Adds to or adjusts the 'page' URL query parameter so that if you follow the
-   * link, you'll get page $index for pager $element on the page.
-   *
-   * The 'page' URL query parameter is a comma-delimited string, where each value
-   * is the target content page for the corresponding pager $element. For
-   * instance, if we have 5 pagers on a single page, and we want to have a link
-   * to a page that should display the 6th content page for the 3rd pager, and
-   * the 1st content page for all the other pagers, then the URL query will look
-   * like this: ?page=0,0,5,0,0 (page numbering starts at zero).
-   *
-   * @param array $query
-   *   An associative array of URL query parameters to add to.
-   * @param int $element
-   *   An integer to distinguish between multiple pagers on one page.
-   * @param int $index
-   *   The index of the target page, for the given element, in the pager array.
-   *
-   * @return array
-   *   The altered $query parameter array.
+   * {@inheritdoc}
    */
   public function queryAddPage(array $query, $element, $index) {
     // Build the 'page' query parameter. This is built based on the current
     // page of each pager element (or NULL if the pager is not set), with the
     // exception of the requested page index for the current element.
     $element_pages = [];
-    for ($i = 0; $i <= $this->getMaxPagerPage(); $i++) {
+    $max = $this->getMaxPagerPage();
+    for ($i = 0; $i <= $max; $i++) {
       $element_pages[] = ($i == $element) ? $index : $this->getPagerPage($i);
     }
     $query['page'] = implode(',', $element_pages);
 
     // Merge the query parameters passed to this function with the parameters
-    // from the current request. In case of collision, the parameters passed into
-    // this function take precedence.
+    // from the current request. In case of collision, the parameters passed
+    // into this function take precedence.
     if ($current_request_query = $this->getQueryParameters()) {
       $query = array_merge($current_request_query, $query);
     }
     return $query;
   }
+
   /**
    * Get the pager page for the given pager element.
    *
    * @param int $element
+   *   The pager element.
    *
    * @return int|null
    *   Page number if present, NULL if not present. Counting starts at 0,
@@ -249,6 +127,9 @@ protected function getPagerPage($element) {
 
   /**
    * Gets the extent of the pager page element IDs.
+   *
+   * @return int
+   *   The maximum element ID available.
    */
   protected function getMaxPagerPage() {
     global $pager_page_array;
@@ -259,6 +140,8 @@ protected function getMaxPagerPage() {
    * Get the total items available for the given pager element.
    *
    * @param int $element
+   *   The pager element.
+   *
    * @return int
    *   The total items available to page through for the given pager element.
    */
@@ -271,7 +154,10 @@ protected function getTotalItems($element) {
    * Get the total for the given pager element.
    *
    * @param int $element
+   *   The pager element.
+   *
    * @return int
+   *   Total for the element.
    */
   protected function getTotal($element) {
     global $pager_total;
@@ -283,7 +169,7 @@ protected function getTotal($element) {
    *
    * @param int $element
    *   The pager element.
-   * @param type $total
+   * @param int $total
    *   The total number value.
    */
   protected function setTotalItems($element, $total) {
@@ -295,7 +181,9 @@ protected function setTotalItems($element, $total) {
    * Set the total for the given pager element.
    *
    * @param int $element
+   *   The pager element.
    * @param int $total
+   *   The total.
    */
   protected function setTotal($element, $total) {
     global $pager_total;
@@ -306,7 +194,9 @@ protected function setTotal($element, $total) {
    * Set the current page for the given pager element.
    *
    * @param int $element
+   *   The pager element.
    * @param int $page
+   *   The new current page.
    */
   protected function setPagerPage($element, $page) {
     global $pager_page_array;
@@ -317,7 +207,7 @@ protected function setPagerPage($element, $page) {
    * Set the number of items displayed per page, for the given pager element.
    *
    * @param int $element
-   *   The element.
+   *   The pager element.
    * @param int $limit
    *   The number of items that will be displayed per page, for the given
    *   element.
diff --git a/core/lib/Drupal/Core/Pager/PagerManagerInterface.php b/core/lib/Drupal/Core/Pager/PagerManagerInterface.php
new file mode 100644
index 0000000000..ff6d9e2233
--- /dev/null
+++ b/core/lib/Drupal/Core/Pager/PagerManagerInterface.php
@@ -0,0 +1,170 @@
+<?php
+
+namespace Drupal\Core\Pager;
+
+/**
+ * This is a service for pager information.
+ *
+ * This class does not deal with pager render elements.
+ *
+ * Since there can be multiple pagers per requested page, each one is
+ * represented by an 'element' ID. This is an integer. It represents the index
+ * of the pager element within the 'page' query. The pager element is an
+ * integer telling us the current page number for that pager.
+ *
+ * This class generally replaces the functions in core/includes/pager.inc. Those
+ * functions use globals to store data which they all use. Since we require
+ * backwards compatibility with this behavior, this class presents a public API
+ * for using pager information, which is implemented using the same globals as a
+ * 'backend.'
+ *
+ * @todo Re-implement the parts of this class that use globals in Drupal 9.
+ */
+interface PagerManagerInterface {
+
+  /**
+   * Initializes a pager.
+   *
+   * This function sets up the necessary variables so that the render system
+   * will correctly process #type 'pager' render arrays to output pagers that
+   * correspond to the items being displayed.
+   *
+   * If the items being displayed result from a database query performed using
+   * Drupal's database API, and if you have control over the construction of the
+   * database query, you do not need to call this function directly; instead,
+   * you can extend the query object with the 'PagerSelectExtender' extender
+   * before executing it. For example:
+   * @code
+   *   $query = db_select('some_table')
+   *     ->extend('Drupal\Core\Database\Query\PagerSelectExtender');
+   * @endcode
+   *
+   * However, if you are using a different method for generating the items to be
+   * paged through, then you should call this function in preparation.
+   *
+   * The following example shows how this function can be used in a controller
+   * that invokes an external datastore with an SQL-like syntax:
+   * @code
+   *   // First find the total number of items and initialize the pager.
+   *   $where = "status = 1";
+   *   $total = mymodule_select("SELECT COUNT(*) FROM data " . $where)->result();
+   *   $num_per_page = \Drupal::config('mymodule.settings')->get('num_per_page');
+   *   $page = \Drupal::service('pager.manager')->defaultInitialize($total, $num_per_page);
+   *
+   *   // Next, retrieve the items for the current page and put them into a
+   *   // render array.
+   *   $offset = $num_per_page * $page;
+   *   $result = mymodule_select("SELECT * FROM data " . $where . " LIMIT %d, %d", $offset, $num_per_page)->fetchAll();
+   *   $render = [];
+   *   $render[] = [
+   *     '#theme' => 'mymodule_results',
+   *     '#result' => $result,
+   *   ];
+   *
+   *   // Finally, add the pager to the render array, and return.
+   *   $render[] = ['#type' => 'pager'];
+   *   return $render;
+   * @endcode
+   *
+   * A second example involves a controller that invokes an external search
+   * service where the total number of matching results is provided as part of
+   * the returned set (so that we do not need a separate query in order to
+   * obtain this information). Here, we call pager_find_page() to calculate the
+   * desired offset before the search is invoked:
+   * @code
+   *   $pager_manager = \Drupal::service('pager.manager');
+   *   // Perform the query, using the requested offset from pager_find_page().
+   *   // This comes from a URL parameter, so here we are assuming that the URL
+   *   // parameter corresponds to an actual page of results that will exist
+   *   // within the set.
+   *   $page = $pager_manager->findPage();
+   *   $num_per_page = \Drupal::config('mymodule.settings')->get('num_per_page');
+   *   $offset = $num_per_page * $page;
+   *   $result = mymodule_remote_search($keywords, $offset, $num_per_page);
+   *
+   *   // Now that we have the total number of results, initialize the pager.
+   *   $pager_manager->defaultInitialize($result->total, $num_per_page);
+   *
+   *   // Create a render array with the search results.
+   *   $render = [];
+   *   $render[] = [
+   *     '#theme' => 'search_results',
+   *     '#results' => $result->data,
+   *     '#type' => 'remote',
+   *   ];
+   *
+   *   // Finally, add the pager to the render array, and return.
+   *   $render[] = ['#type' => 'pager'];
+   *   return $render;
+   * @endcode
+   *
+   * @param int $total
+   *   The total number of items to be paged.
+   * @param int $limit
+   *   The number of items the calling code will display per page.
+   * @param int $element
+   *   (optional) An integer to distinguish between multiple pagers on one page.
+   *
+   * @return int
+   *   The number of the current page, within the pager represented by $element.
+   *   This is determined from the URL query parameter
+   *   \Drupal::request()->query->get('page), or 0 by default. However, if a
+   *   page that does not correspond to the actual range of the result set was
+   *   requested, this function will return the closest page actually within the
+   *   result set.
+   */
+  public function defaultInitialize($total, $limit, $element = 0);
+
+  /**
+   * Returns the current page being requested for display within a pager.
+   *
+   * @param int $element
+   *   (optional) An integer to distinguish between multiple pagers on one page.
+   *
+   * @return int
+   *   The number of the current requested page, within the pager represented by
+   *   $element. This is determined from the URL query parameter
+   *   \Drupal::request()->query->get('page'), or 0 by default. Note that this
+   *   number may differ from the actual page being displayed. For example, if a
+   *   search for "example text" brings up three pages of results, but a user
+   *   visits search/node/example+text?page=10, this function will return 10,
+   *   even though the default pager implementation adjusts for this and still
+   *   displays the third page of search results at that URL.
+   */
+  public function findPage($element = 0);
+
+  /**
+   * Compose a URL query parameter array for pager links.
+   *
+   * @return array
+   *   A URL query parameter array that consists of all components of the
+   *   current page request except for those pertaining to paging.
+   */
+  public function getQueryParameters();
+
+  /**
+   * Gets the URL query parameter array of a pager link.
+   *
+   * Adds to or adjusts the 'page' URL query parameter so that if you follow the
+   * link, you'll get page $index for pager $element on the page.
+   *
+   * The 'page' URL query parameter is a comma-delimited string, where each
+   * value is the target content page for the corresponding pager $element. For
+   * instance, if we have 5 pagers on a single page, and we want to have a link
+   * to a page that should display the 6th content page for the 3rd pager, and
+   * the 1st content page for all the other pagers, then the URL query will look
+   * like this: ?page=0,0,5,0,0 (page numbering starts at zero).
+   *
+   * @param array $query
+   *   An associative array of URL query parameters to add to.
+   * @param int $element
+   *   An integer to distinguish between multiple pagers on one page.
+   * @param int $index
+   *   The index of the target page, for the given element, in the pager array.
+   *
+   * @return array
+   *   The altered $query parameter array.
+   */
+  public function queryAddPage(array $query, $element, $index);
+
+}
diff --git a/core/tests/Drupal/KernelTests/Core/Pager/PagerManagerTest.php b/core/tests/Drupal/KernelTests/Core/Pager/PagerManagerTest.php
new file mode 100644
index 0000000000..f22f969446
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Pager/PagerManagerTest.php
@@ -0,0 +1,96 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Pager;
+
+use Drupal\Core\Pager\PagerManager;
+use Drupal\KernelTests\KernelTestBase;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * @group Pager
+ *
+ * @coversDefaultClass \Drupal\Core\Pager\PagerManager
+ */
+class PagerManagerTest extends KernelTestBase {
+
+  /**
+   * @covers ::defaultInitialize
+   */
+  public function testDefaultInitializeGlobals() {
+    $pager_globals = [
+      'pager_page_array',
+      'pager_total_items',
+      'pager_total',
+      'pager_limits',
+    ];
+    foreach ($pager_globals as $pager_global) {
+      $this->assertFalse(isset($GLOBALS[$pager_global]));
+    }
+
+    $pm = new PagerManager($this->container->get('request_stack'));
+    $pm->defaultInitialize(5, 1);
+
+    foreach ($pager_globals as $pager_global) {
+      $this->assertTrue(isset($GLOBALS[$pager_global]));
+    }
+  }
+
+  /**
+   * @covers ::findPage
+   */
+  public function testFindPage() {
+    $request = Request::create('http://example.com', 'GET', ['page' => '0,10']);
+
+    /* @var $request_stack \Symfony\Component\HttpFoundation\RequestStack */
+    $request_stack = $this->container->get('request_stack');
+    $request_stack->push($request);
+
+    $pm = new PagerManager($request_stack);
+
+    $this->assertEquals(10, $pm->findPage(1));
+  }
+
+  /**
+   * @covers ::getQueryParameters
+   */
+  public function testGetQueryParameters() {
+    $test_parameters = [
+      'other' => 'arbitrary',
+    ];
+    $request = Request::create('http://example.com', 'GET', array_merge(['page' => '0,10'], $test_parameters));
+
+    /* @var $request_stack \Symfony\Component\HttpFoundation\RequestStack */
+    $request_stack = $this->container->get('request_stack');
+    $request_stack->push($request);
+
+    $pm = new PagerManager($request_stack);
+
+    $this->assertEquals($test_parameters, $pm->getQueryParameters());
+  }
+
+  /**
+   * @covers ::queryAddPage
+   */
+  public function testQueryAddPage() {
+    $element = 2;
+    $index = 5;
+    $test_parameters = [
+      'other' => 'arbitrary',
+    ];
+    $request = Request::create('http://example.com', 'GET', $test_parameters);
+
+    /* @var $request_stack \Symfony\Component\HttpFoundation\RequestStack */
+    $request_stack = $this->container->get('request_stack');
+    $request_stack->push($request);
+
+    $pm = new PagerManager($request_stack);
+    $pm->defaultInitialize(30, 10, $element);
+
+    $query = $pm->queryAddPage($request->query->all(), $element, $index);
+
+    $this->assertArrayHasKey('other', $query);
+
+    $this->assertEquals(",,$index", $query['page']);
+  }
+
+}