diff --git a/inc/Subjects.php b/inc/Subjects.php
index 4d2cfdb..1578398 100644
--- a/inc/Subjects.php
+++ b/inc/Subjects.php
@@ -11,4 +11,71 @@
 
     return $query->fetchAll(\PDO::FETCH_ASSOC);
   }
+
+  public static function exists(int $subject): bool {
+    global $con;
+    $query = $con->prepare('SELECT id FROM subjects WHERE id = ?');
+    if (!$query->execute([$subject]))
+      return false;
+
+    return $query->rowCount() > 0;
+  }
+
+  public static function isNewUserSubject(int $subjectId): bool {
+    global $con;
+    $query = $con->prepare('SELECT id FROM user_subjects WHERE user_id = :user_id AND subject_id = :subject_id');
+
+    $userId = Users::getUserId();
+    if ($userId == -1 || !$query->execute([
+      'user_id' => $userId,
+      'subject_id' => $subjectId
+    ]))
+      return false;
+
+    return $query->rowCount() == 0;
+  }
+
+  public static function getUserSubjects() {
+    global $con;
+    $query = $con->prepare('SELECT
+        us.id, us.subject_id, s.friendly_name, s.calendar_name
+        FROM user_subjects us
+        INNER JOIN subjects s
+          ON us.subject_id = s.id
+        WHERE us.user_id = ?');
+
+    $userId = Users::getUserId();
+    if ($userId == -1 || !$query->execute([$userId]))
+      return false;
+
+    return $query->fetchAll(\PDO::FETCH_ASSOC);
+  }
+
+  public static function addUserSubject(int $subjectId): bool {
+    global $con;
+
+    if (!self::exists($subjectId) || !self::isNewUserSubject($subjectId))
+      return false;
+
+    $query = $con->prepare('INSERT INTO user_subjects (user_id, subject_id) VALUES (:user_id, :subject_id)');
+
+    $userId = Users::getUserId();
+
+    return $userId != -1 && $query->execute([
+      'user_id' => $userId,
+      'subject_id' => $subjectId
+    ]);
+  }
+
+  public static function removeUserSubject(int $subjectId): bool {
+    global $con;
+
+    $query = $con->prepare('DELETE FROM user_subjects WHERE user_id = :user_id AND subject_id = :subject_id LIMIT 1');
+
+    $userId = Users::getUserId();
+    return $userId != -1 && $query->execute([
+      'user_id' => $userId,
+      'subject_id' => $subjectId
+    ]);
+  }
 }