diff --git a/src/lti/LTI_Course_Groups_Service.php b/src/lti/LTI_Course_Groups_Service.php
new file mode 100644
index 00000000..b9a878c3
--- /dev/null
+++ b/src/lti/LTI_Course_Groups_Service.php
@@ -0,0 +1,110 @@
+<?php
+namespace IMSGlobal\LTI;
+
+class LTI_Course_Groups_Service {
+
+    private $service_connector;
+    private $service_data;
+
+    public function __construct(LTI_Service_Connector $service_connector, $service_data) {
+        $this->service_connector = $service_connector;
+        $this->service_data = $service_data;
+    }
+
+    public function get_groups() {
+
+        $groups = [];
+
+        $next_page = $this->service_data['context_groups_url'];
+
+        while ($next_page) {
+            $page = $this->service_connector->make_service_request(
+                $this->service_data['scope'],
+                'GET',
+                $next_page,
+                null,
+                null,
+                'application/vnd.ims.lti-gs.v1.contextgroupcontainer+json'
+            );
+
+            $groups = array_merge($groups, $page['body']['groups']);
+
+            $next_page = false;
+            foreach($page['headers'] as $header) {
+                if (preg_match(LTI_Service_Connector::NEXT_PAGE_REGEX, $header, $matches)) {
+                    $next_page = $matches[1];
+                    break;
+                }
+            }
+        }
+        return $groups;
+
+    }
+
+    public function get_sets() {
+
+        $sets = [];
+
+        // Sets are optional.
+        if (!isset($this->service_data['context_group_sets_url'])) {
+            return [];
+        }
+
+        $next_page = $this->service_data['context_group_sets_url'];
+
+        while ($next_page) {
+            $page = $this->service_connector->make_service_request(
+                $this->service_data['scope'],
+                'GET',
+                $next_page,
+                null,
+                null,
+                'application/vnd.ims.lti-gs.v1.contextgroupcontainer+json'
+            );
+
+            $sets = array_merge($sets, $page['body']['sets']);
+
+            $next_page = false;
+            foreach($page['headers'] as $header) {
+                if (preg_match(LTI_Service_Connector::NEXT_PAGE_REGEX, $header, $matches)) {
+                    $next_page = $matches[1];
+                    break;
+                }
+            }
+        }
+        return $sets;
+
+    }
+
+    public function get_groups_by_set() {
+        $groups = $this->get_groups();
+        $sets = $this->get_sets();
+
+        $groups_by_set = [];
+        $unsetted = [];
+
+        foreach ($sets as $key => $set) {
+            $groups_by_set[$set['id']] = $set;
+            $groups_by_set[$set['id']]['groups'] = [];
+        }
+
+        foreach ($groups as $key => $group) {
+            if (isset($group['set_id']) && isset($groups_by_set[$group['set_id']])) {
+                $groups_by_set[$group['set_id']]['groups'][$group['id']] = $group;
+            } else {
+                $unsetted[$group['id']] = $group;
+            }
+        }
+
+        if (!empty($unsetted)) {
+            $groups_by_set['none'] = [
+                "name" => "None",
+                "id" => "none",
+                "groups" => $unsetted,
+            ];
+        }
+
+        return $groups_by_set;
+    }
+}
+?>
\ No newline at end of file
diff --git a/src/lti/LTI_Grade.php b/src/lti/LTI_Grade.php
index 75e7ab2a..fabeb6ad 100644
--- a/src/lti/LTI_Grade.php
+++ b/src/lti/LTI_Grade.php
@@ -4,10 +4,12 @@
 class LTI_Grade {
     private $score_given;
     private $score_maximum;
+    private $comment;
     private $activity_progress;
     private $grading_progress;
     private $timestamp;
     private $user_id;
+    private $submission_review;
 
     /**
      * Static function to allow for method chaining without having to assign to a variable first.
@@ -34,6 +36,15 @@ public function set_score_maximum($value) {
         return $this;
     }
 
+    public function get_comment() {
+        return $this->comment;
+    }
+
+    public function set_comment($comment) {
+        $this->comment = $comment;
+        return $this;
+    }
+
     public function get_activity_progress() {
         return $this->activity_progress;
     }
@@ -70,14 +81,25 @@ public function set_user_id($value) {
         return $this;
     }
 
+    public function get_submission_review() {
+        return $this->submission_review;
+    }
+
+    public function set_submission_review($value) {
+        $this->submission_review = $value;
+        return $this;
+    }
+
     public function __toString() {
         return json_encode(array_filter([
             "scoreGiven" => 0 + $this->score_given,
             "scoreMaximum" => 0 + $this->score_maximum,
+            "comment" => $this->comment,
             "activityProgress" => $this->activity_progress,
             "gradingProgress" => $this->grading_progress,
             "timestamp" => $this->timestamp,
             "userId" => $this->user_id,
+            "submissionReview" => $this->submission_review,
         ]));
     }
 }
diff --git a/src/lti/LTI_Grade_Submission_Review.php b/src/lti/LTI_Grade_Submission_Review.php
new file mode 100644
index 00000000..73fc073f
--- /dev/null
+++ b/src/lti/LTI_Grade_Submission_Review.php
@@ -0,0 +1,62 @@
+<?php
+namespace IMSGlobal\LTI;
+
+class LTI_Grade_Submission_Review {
+    private $reviewable_status;
+    private $label;
+    private $url;
+    private $custom;
+
+    /**
+     * Static function to allow for method chaining without having to assign to a variable first.
+     */
+    public static function new() {
+        return new LTI_Grade_Submission_Review();
+    }
+
+    public function get_reviewable_status() {
+        return $this->reviewable_status;
+    }
+
+    public function set_reviewable_status($value) {
+        $this->reviewable_status = $value;
+        return $this;
+    }
+
+    public function get_label() {
+        return $this->label;
+    }
+
+    public function set_label($value) {
+        $this->label = $value;
+        return $this;
+    }
+
+    public function get_url() {
+        return $this->url;
+    }
+
+    public function set_url($url) {
+        $this->url = $url;
+        return $this;
+    }
+
+    public function get_custom() {
+        return $this->custom;
+    }
+
+    public function set_custom($value) {
+        $this->custom = $value;
+        return $this;
+    }
+
+    public function __toString() {
+        return json_encode(array_filter([
+            "reviewableStatus" => $this->reviewable_status,
+            "label" => $this->label,
+            "url" => $this->url,
+            "custom" => $this->custom,
+        ]));
+    }
+}
+?>
\ No newline at end of file
diff --git a/src/lti/LTI_Message_Launch.php b/src/lti/LTI_Message_Launch.php
index 5dd9b6f1..7a18195d 100644
--- a/src/lti/LTI_Message_Launch.php
+++ b/src/lti/LTI_Message_Launch.php
@@ -108,6 +108,26 @@ public function get_nrps() {
             $this->jwt['body']['https://purl.imsglobal.org/spec/lti-nrps/claim/namesroleservice']);
     }
 
+    /**
+     * Returns whether or not the current launch can use the groups service.
+     *
+     * @return boolean  Returns a boolean indicating the availability of groups.
+     */
+    public function has_gs() {
+        return !empty($this->jwt['body']['https://purl.imsglobal.org/spec/lti-gs/claim/groupsservice']['context_groups_url']);
+    }
+
+    /**
+     * Fetches an instance of the groups service for the current launch.
+     *
+     * @return LTI_Course_Groups_Service An instance of the groups service that can be used to make calls within the scope of the current launch.
+     */
+    public function get_gs() {
+        return new LTI_Course_Groups_Service(
+            new LTI_Service_Connector($this->registration),
+            $this->jwt['body']['https://purl.imsglobal.org/spec/lti-gs/claim/groupsservice']);
+    }
+
     /**
      * Returns whether or not the current launch can use the assignments and grades service.
      *
@@ -149,6 +169,15 @@ public function is_deep_link_launch() {
         return $this->jwt['body']['https://purl.imsglobal.org/spec/lti/claim/message_type'] === 'LtiDeepLinkingRequest';
     }
 
+    /**
+     * Returns whether or not the current launch is a submission review launch.
+     *
+     * @return boolean  Returns true if the current launch is a submission review launch.
+     */
+    public function is_submission_review_launch() {
+        return $this->jwt['body']['https://purl.imsglobal.org/spec/lti/claim/message_type'] === 'LtiSubmissionReviewRequest';
+    }
+
     /**
      * Returns whether or not the current launch is a resource launch.
      *
diff --git a/src/lti/LTI_Names_Roles_Provisioning_Service.php b/src/lti/LTI_Names_Roles_Provisioning_Service.php
index 86df669c..df2242ea 100644
--- a/src/lti/LTI_Names_Roles_Provisioning_Service.php
+++ b/src/lti/LTI_Names_Roles_Provisioning_Service.php
@@ -31,7 +31,7 @@ public function get_members() {
 
             $next_page = false;
             foreach($page['headers'] as $header) {
-                if (preg_match("/^Link:.*<([^>]*)>; ?rel=\"next\"/i", $header, $matches)) {
+                if (preg_match(LTI_Service_Connector::NEXT_PAGE_REGEX, $header, $matches)) {
                     $next_page = $matches[1];
                     break;
                 }
diff --git a/src/lti/LTI_Service_Connector.php b/src/lti/LTI_Service_Connector.php
index 1c8d9115..cd2318e5 100644
--- a/src/lti/LTI_Service_Connector.php
+++ b/src/lti/LTI_Service_Connector.php
@@ -4,6 +4,9 @@
 use Firebase\JWT\JWT;
 
 class LTI_Service_Connector {
+
+    const NEXT_PAGE_REGEX = "/^Link:.*<([^>]*)>; ?rel=\"next\"/i";
+
     private $registration;
     private $access_tokens = [];
 
diff --git a/src/lti/message_validators/submission_review_message_validator.php b/src/lti/message_validators/submission_review_message_validator.php
new file mode 100644
index 00000000..60cc4914
--- /dev/null
+++ b/src/lti/message_validators/submission_review_message_validator.php
@@ -0,0 +1,29 @@
+<?php
+namespace IMSGlobal\LTI;
+
+class Submission_Review_Message_Validator implements Message_Validator {
+    public function can_validate($jwt_body) {
+        return $jwt_body['https://purl.imsglobal.org/spec/lti/claim/message_type'] === 'LtiSubmissionReviewRequest';
+    }
+
+    public function validate($jwt_body) {
+        if (empty($jwt_body['sub'])) {
+            throw new LTI_Exception('Must have a user (sub)');
+        }
+        if ($jwt_body['https://purl.imsglobal.org/spec/lti/claim/version'] !== '1.3.0') {
+            throw new LTI_Exception('Incorrect version, expected 1.3.0');
+        }
+        if (!isset($jwt_body['https://purl.imsglobal.org/spec/lti/claim/roles'])) {
+            throw new LTI_Exception('Missing Roles Claim');
+        }
+        if (empty($jwt_body['https://purl.imsglobal.org/spec/lti/claim/resource_link']['id'])) {
+            throw new LTI_Exception('Missing Resource Link Id');
+        }
+        if (empty($jwt_body['https://purl.imsglobal.org/spec/lti/claim/for_user'])) {
+            throw new LTI_Exception('Missing For User');
+        }
+
+        return true;
+    }
+}
+?>
\ No newline at end of file