diff --git a/lang/en/cache.php b/lang/en/cache.php
index 5fac49e8984..3e633792143 100644
--- a/lang/en/cache.php
+++ b/lang/en/cache.php
@@ -43,6 +43,7 @@ $string['cachedef_coursecatrecords'] = 'Course categories records';
 $string['cachedef_coursecontacts'] = 'List of course contacts';
 $string['cachedef_coursecattree'] = 'Course categories tree';
 $string['cachedef_coursemodinfo'] = 'Accumulated information about modules and sections for each course';
+$string['cachedef_completion'] = 'Activity completion status';
 $string['cachedef_databasemeta'] = 'Database meta information';
 $string['cachedef_eventinvalidation'] = 'Event invalidation';
 $string['cachedef_externalbadges'] = 'External badges for particular user';
@@ -173,4 +174,4 @@ $string['userinputsharingkey_help'] = 'Enter your own private key here. When you
 // Deprecated since 2.9.
 $string['lockingmeans'] = 'Locking mechanism';
 $string['lockmethod'] = 'Lock method';
-$string['lockmethod_help'] = 'This is the method used for locking when required of this store.';
\ No newline at end of file
+$string['lockmethod_help'] = 'This is the method used for locking when required of this store.';
diff --git a/lib/completionlib.php b/lib/completionlib.php
index 2e83745084b..b7af0b2783b 100644
--- a/lib/completionlib.php
+++ b/lib/completionlib.php
@@ -119,12 +119,6 @@ define('COMPLETION_VIEWED', 1);
  */
 define('COMPLETION_NOT_VIEWED', 0);
 
-/**
- * Cache expiry time in seconds (10 minutes)
- * Completion cacheing
- */
-define('COMPLETION_CACHE_EXPIRY', 10*60);
-
 /**
  * Completion details should be ORed together and you should return false if
  * none apply.
@@ -553,7 +547,7 @@ class completion_info {
      * @return void
      */
     public function update_state($cm, $possibleresult=COMPLETION_UNKNOWN, $userid=0) {
-        global $USER, $SESSION;
+        global $USER;
 
         // Do nothing if completion is not enabled for that activity
         if (!$this->is_enabled($cm)) {
@@ -792,20 +786,13 @@ class completion_info {
      * Used by course reset page.
      */
     public function delete_all_completion_data() {
-        global $DB, $SESSION;
+        global $DB;
 
         // Delete from database.
         $DB->delete_records_select('course_modules_completion',
                 'coursemoduleid IN (SELECT id FROM {course_modules} WHERE course=?)',
                 array($this->course_id));
 
-        // Reset cache for current user.
-        if (isset($SESSION->completioncache) &&
-            array_key_exists($this->course_id, $SESSION->completioncache)) {
-
-            unset($SESSION->completioncache[$this->course_id]);
-        }
-
         // Wipe course completion data too.
         $this->delete_course_completion_data();
     }
@@ -818,19 +805,11 @@ class completion_info {
      * @param stdClass|cm_info $cm Activity
      */
     public function delete_all_state($cm) {
-        global $SESSION, $DB;
+        global $DB;
 
         // Delete from database
         $DB->delete_records('course_modules_completion', array('coursemoduleid'=>$cm->id));
 
-        // Erase cache data for current user if applicable
-        if (isset($SESSION->completioncache) &&
-            array_key_exists($cm->course, $SESSION->completioncache) &&
-            array_key_exists($cm->id, $SESSION->completioncache[$cm->course])) {
-
-            unset($SESSION->completioncache[$cm->course][$cm->id]);
-        }
-
         // Check if there is an associated course completion criteria
         $criteria = $this->get_criteria(COMPLETION_CRITERIA_TYPE_ACTIVITY);
         $acriteria = false;
@@ -876,7 +855,7 @@ class completion_info {
         }
         $rs->close();
 
-        // Delete all existing state [also clears session cache for current user]
+        // Delete all existing state.
         $this->delete_all_state($cm);
 
         // Merge this with list of planned users (according to roles)
@@ -894,7 +873,7 @@ class completion_info {
 
     /**
      * Obtains completion data for a particular activity and user (from the
-     * session cache if available, or by SQL query)
+     * completion cache if available, or by SQL query)
      *
      * @param stcClass|cm_info $cm Activity; only required field is ->id
      * @param bool $wholecourse If true (default false) then, when necessary to
@@ -908,39 +887,21 @@ class completion_info {
      * @return object Completion data (record from course_modules_completion)
      */
     public function get_data($cm, $wholecourse = false, $userid = 0, $modinfo = null) {
-        global $USER, $CFG, $SESSION, $DB;
+        global $USER, $CFG, $DB;
+        $completioncache = cache::make('core', 'completion');
 
         // Get user ID
         if (!$userid) {
             $userid = $USER->id;
         }
 
-        // Is this the current user?
-        $currentuser = $userid==$USER->id;
-
-        if ($currentuser && is_object($SESSION)) {
-            // Make sure cache is present and is for current user (loginas
-            // changes this)
-            if (!isset($SESSION->completioncache) || $SESSION->completioncacheuserid!=$USER->id) {
-                $SESSION->completioncache = array();
-                $SESSION->completioncacheuserid = $USER->id;
-            }
-            // Expire any old data from cache
-            foreach ($SESSION->completioncache as $courseid=>$activities) {
-                if (empty($activities['updated']) || $activities['updated'] < time()-COMPLETION_CACHE_EXPIRY) {
-                    unset($SESSION->completioncache[$courseid]);
-                }
-            }
-            // See if requested data is present, if so use cache to get it
-            if (isset($SESSION->completioncache) &&
-                array_key_exists($this->course->id, $SESSION->completioncache) &&
-                array_key_exists($cm->id, $SESSION->completioncache[$this->course->id])) {
-                return $SESSION->completioncache[$this->course->id][$cm->id];
-            }
+        // See if requested data is present in cache
+        if ($cacheddata = $completioncache->get($userid . '_' . $this->course->id . '_' . $cm->id)) {
+            return $cacheddata;
         }
 
         // Not there, get via SQL
-        if ($currentuser && $wholecourse) {
+        if ($wholecourse) {
             // Get whole course data for cache
             $alldatabycmc = $DB->get_records_sql("
     SELECT
@@ -976,14 +937,13 @@ class completion_info {
                     $data->viewed          = 0;
                     $data->timemodified    = 0;
                 }
-                $SESSION->completioncache[$this->course->id][$othercm->id] = $data;
+                $completioncache->set($userid . '_' . $this->course->id . '_' . $othercm->id, $data);
             }
-            $SESSION->completioncache[$this->course->id]['updated'] = time();
 
-            if (!isset($SESSION->completioncache[$this->course->id][$cm->id])) {
+            if (!$completiondata = $completioncache->get($userid . '_' . $this->course->id . '_' . $cm->id)) {
                 $this->internal_systemerror("Unexpected error: course-module {$cm->id} could not be found on course {$this->course->id}");
             }
-            return $SESSION->completioncache[$this->course->id][$cm->id];
+            return $completiondata;
 
         } else {
             // Get single record
@@ -1000,13 +960,7 @@ class completion_info {
             }
 
             // Put in cache
-            if ($currentuser) {
-                $SESSION->completioncache[$this->course->id][$cm->id] = $data;
-                // For single updates, only set date if it was empty before
-                if (empty($SESSION->completioncache[$this->course->id]['updated'])) {
-                    $SESSION->completioncache[$this->course->id]['updated'] = time();
-                }
-            }
+            $completioncache->set($userid . '_' . $this->course->id . '_' . $cm->id, $data);
         }
 
         return $data;
@@ -1022,7 +976,7 @@ class completion_info {
      * @param stdClass $data Data about completion for that user
      */
     public function internal_set_data($cm, $data) {
-        global $USER, $SESSION, $DB;
+        global $USER, $DB;
 
         $transaction = $DB->start_delegated_transaction();
         if (!$data->id) {
@@ -1054,8 +1008,10 @@ class completion_info {
         $event->add_record_snapshot('course_modules_completion', $data);
         $event->trigger();
 
+        $completioncache = cache::make('core', 'completion');
+        $completioncache->set($data->userid . '_' . $cm->course . '_' . $cm->id, $data);
+        // TODO under what circumstances should I call get_fast_modinfo()?
         if ($data->userid == $USER->id) {
-            $SESSION->completioncache[$cm->course][$cm->id] = $data;
             // reset modinfo for user (no need to call rebuild_course_cache())
             get_fast_modinfo($cm->course, 0, true);
         }
@@ -1341,13 +1297,4 @@ class completion_info {
         throw new moodle_exception('err_system','completion',
             $CFG->wwwroot.'/course/view.php?id='.$this->course->id,null,$error);
     }
-
-    /**
-     * For testing only. Wipes information cached in user session.
-     */
-    public static function wipe_session_cache() {
-        global $SESSION;
-        unset($SESSION->completioncache);
-        unset($SESSION->completioncacheuserid);
-    }
 }
diff --git a/lib/db/caches.php b/lib/db/caches.php
index e71dc577fee..2b08e21f4c2 100644
--- a/lib/db/caches.php
+++ b/lib/db/caches.php
@@ -213,6 +213,13 @@ $definitions = array(
         'ttl' => 3600,
     ),
 
+    // Used to cache activity completion status.
+    'completion' => array(
+        'mode' => cache_store::MODE_APPLICATION,
+        'simplekeys' => true,
+        'ttl' => 10*60,
+    ),
+
     // A simple cache that stores whether a user can expand a course in the navigation.
     // The key is the course ID and the value will either be 1 or 0 (cast to bool).
     // The cache isn't always up to date, it should only ever be used to save a costly call to
diff --git a/lib/tests/completionlib_test.php b/lib/tests/completionlib_test.php
index 150bafc789d..927ae65c6c2 100644
--- a/lib/tests/completionlib_test.php
+++ b/lib/tests/completionlib_test.php
@@ -325,7 +325,7 @@ class core_completionlib_testcase extends advanced_testcase {
     }
 
     public function test_delete_all_state() {
-        global $DB, $SESSION;
+        global $DB;
         $this->mock_setup();
 
         $course = (object)array('id'=>13);
@@ -339,21 +339,6 @@ class core_completionlib_testcase extends advanced_testcase {
             ->with('course_modules_completion', array('coursemoduleid'=>42))
             ->will($this->returnValue(true));
         $c->delete_all_state($cm);
-
-        // Build up a session to check it deletes the right bits from it
-        // (and not other bits).
-        $SESSION->completioncache = array();
-        $SESSION->completioncache[13] = array();
-        $SESSION->completioncache[13][42] = 'foo';
-        $SESSION->completioncache[13][43] = 'foo';
-        $SESSION->completioncache[14] = array();
-        $SESSION->completioncache[14][42] = 'foo';
-        $DB->expects($this->at(0))
-            ->method('delete_records')
-            ->with('course_modules_completion', array('coursemoduleid'=>42))
-            ->will($this->returnValue(true));
-        $c->delete_all_state($cm);
-        $this->assertEquals(array(13=>array(43=>'foo'), 14=>array(42=>'foo')), $SESSION->completioncache);
     }
 
     public function test_reset_all_state() {
@@ -396,9 +381,11 @@ class core_completionlib_testcase extends advanced_testcase {
     }
 
     public function test_get_data() {
-        global $DB, $SESSION;
+        global $DB;
         $this->mock_setup();
 
+        $cache = cache::make('core', 'completion');
+
         $c = new completion_info((object)array('id'=>42));
         $cm = (object)array('id'=>13, 'course'=>42);
 
@@ -412,18 +399,20 @@ class core_completionlib_testcase extends advanced_testcase {
             ->will($this->returnValue($sillyrecord));
         $result = $c->get_data($cm, false, 123);
         $this->assertEquals($sillyrecord, $result);
-        $this->assertFalse(isset($SESSION->completioncache));
+        $this->assertEquals($cache->get('123_42_13'), $sillyrecord);
 
-        // 2. Not current user, default record, whole course (ignored).
+        // 2. Not current user, default record, whole course.
+        $cache->purge();
         $DB->expects($this->at(0))
-            ->method('get_record')
-            ->with('course_modules_completion', array('coursemoduleid'=>13, 'userid'=>123))
-            ->will($this->returnValue(false));
-        $result=$c->get_data($cm, true, 123);
+            ->method('get_records_sql')
+            ->will($this->returnValue(array()));
+        $modinfo = new stdClass();
+        $modinfo->cms = array((object)array('id'=>13));
+        $result=$c->get_data($cm, true, 123, $modinfo);
         $this->assertEquals((object)array(
             'id'=>'0', 'coursemoduleid'=>13, 'userid'=>123, 'completionstate'=>0,
             'viewed'=>0, 'timemodified'=>0), $result);
-        $this->assertFalse(isset($SESSION->completioncache));
+        $this->assertEquals($cache->get('123_42_13'), $result);
 
         // 3. Current user, single record, not from cache.
         $DB->expects($this->at(0))
@@ -432,34 +421,14 @@ class core_completionlib_testcase extends advanced_testcase {
             ->will($this->returnValue($sillyrecord));
         $result = $c->get_data($cm);
         $this->assertEquals($sillyrecord, $result);
-        $this->assertEquals($sillyrecord, $SESSION->completioncache[42][13]);
-        // When checking time(), allow for second overlaps.
-        $this->assertTrue(time()-$SESSION->completioncache[42]['updated']<2);
+        $this->assertEquals($sillyrecord, $cache->get('314159_42_13'));
 
         // 4. Current user, 'whole course', but from cache.
         $result = $c->get_data($cm, true);
         $this->assertEquals($sillyrecord, $result);
 
-        // 5. Current user, single record, cache expired
-        $SESSION->completioncache[42]['updated']=37; // Quite a long time ago.
-        $now = time();
-        $SESSION->completioncache[17]['updated']=$now;
-        $SESSION->completioncache[39]['updated']=72; // Also a long time ago.
-        $DB->expects($this->at(0))
-            ->method('get_record')
-            ->with('course_modules_completion', array('coursemoduleid'=>13, 'userid'=>314159))
-            ->will($this->returnValue($sillyrecord));
-        $result = $c->get_data($cm, false);
-        $this->assertEquals($sillyrecord, $result);
-
-        // Check that updated value is right, then fudge it to make next compare work.
-        $this->assertTrue(time()-$SESSION->completioncache[42]['updated']<2);
-        $SESSION->completioncache[42]['updated']=$now;
-        // Check things got expired from cache.
-        $this->assertEquals(array(42=>array(13=>$sillyrecord, 'updated'=>$now), 17=>array('updated'=>$now)), $SESSION->completioncache);
-
-        // 6. Current user, 'whole course' and record not in cache.
-        unset($SESSION->completioncache);
+        // 5. Current user, 'whole course' and record not in cache.
+        $cache->purge();
 
         // Scenario: Completion data exists for one CMid.
         $basicrecord = (object)array('coursemoduleid'=>13);
@@ -476,15 +445,14 @@ class core_completionlib_testcase extends advanced_testcase {
         $this->assertEquals($basicrecord, $result);
 
         // Check the cache contents.
-        $this->assertTrue(time()-$SESSION->completioncache[42]['updated']<2);
-        $SESSION->completioncache[42]['updated'] = $now;
-        $this->assertEquals(array(42=>array(13=>$basicrecord, 14=>(object)array(
-            'id'=>'0', 'coursemoduleid'=>14, 'userid'=>314159, 'completionstate'=>0,
-            'viewed'=>0, 'timemodified'=>0), 'updated'=>$now)), $SESSION->completioncache);
+        $this->assertEquals($basicrecord, $cache->get('314159_42_13'));
+        $this->assertEquals((object)array('id'=>'0', 'coursemoduleid'=>14, 
+            'userid'=>314159, 'completionstate'=>0, 'viewed'=>0, 'timemodified'=>0),
+            $cache->get('314159_42_14'));
     }
 
     public function test_internal_set_data() {
-        global $DB, $SESSION;
+        global $DB;
         $this->setup_data();
 
         $this->setUser($this->user);
@@ -505,10 +473,11 @@ class core_completionlib_testcase extends advanced_testcase {
         $c->internal_set_data($cm, $data);
         $d1 = $DB->get_field('course_modules_completion', 'id', array('coursemoduleid' => $cm->id));
         $this->assertEquals($d1, $data->id);
-        $this->assertEquals(array($this->course->id => array($cm->id => $data)), $SESSION->completioncache);
+        $cache = cache::make('core', 'completion');
+        $this->assertEquals($cache->get($data->userid . '_' . $cm->course . '_' . $cm->id),
+            $data);
 
-        // 2) Test with existing data and for different user (not cached).
-        unset($SESSION->completioncache);
+        // 2) Test with existing data and for different user.
         $forum2 = $this->getDataGenerator()->create_module('forum', array('course' => $this->course->id), $completionauto);
         $cm2 = get_coursemodule_from_instance('forum', $forum2->id);
         $newuser = $this->getDataGenerator()->create_user();
@@ -521,7 +490,10 @@ class core_completionlib_testcase extends advanced_testcase {
         $d2->timemodified = time();
         $d2->viewed = COMPLETION_NOT_VIEWED;
         $c->internal_set_data($cm2, $d2);
-        $this->assertFalse(isset($SESSION->completioncache));
+        $this->assertEquals($cache->get($data->userid . '_' . $cm->course . '_' . $cm->id),
+            $data);
+        $this->assertEquals($cache->get($d2->userid . '_' . $cm2->course . '_' . $cm2->id),
+            $d2);
 
         // 3) Test where it THINKS the data is new (from cache) but actually
         //    in the database it has been set since.