diff --git a/inc/API.php b/inc/API.php
new file mode 100644
index 0000000..0d5530e
--- /dev/null
+++ b/inc/API.php
@@ -0,0 +1,104 @@
+<?php
+namespace DAFME\Covid;
+
+class API {
+  private static function returnJSON($array) {
+    echo json_encode($array);
+  }
+
+  public static function returnError($errorMessage = 'Unexpected error') {
+    http_response_code(400);
+    self::returnJson([
+      'status' => 'error',
+      'errorMessage' =>  $errorMessage
+    ]);
+  }
+
+  public static function returnPayload($payload) {
+    self::returnJson([
+      'status' => 'ok',
+      'payload' => $payload
+    ]);
+  }
+
+  public static function returnOk() {
+    self::returnJson([
+      'status' => 'ok'
+    ]);
+  }
+
+  private static function checkSignInStatus() {
+    if (!Users::isSignedIn()) {
+      self::returnError('The user hasn\'t signed in.');
+      exit();
+    }    
+  }
+
+  public static function process($path) {
+    global $conf;
+
+    header('Content-Type: application/json');
+
+    if (isset($conf['frontendUrl']) && !empty($conf['frontendUrl']))
+      header('Access-Control-Allow-Origin: '.$conf['frontendUrl']);
+
+    $parts = explode('/', $path);
+    $method = $parts[0] ?? '';
+
+    switch ($method) {
+      case 'getAuthUrl':
+        $auth = new Auth();
+        self::returnPayload([
+          'url' => $auth->getAuthUrl()
+        ]);
+        break;
+
+      case 'isSignedIn':
+        $isSignedIn = \DAFME\Covid\Users::isSignedIn();
+        self::returnPayload([
+          'signedIn' => $isSignedIn
+        ]);
+        break;
+
+      case 'signOut':
+        \DAFME\Covid\Users::signOut();
+        self::returnOk();
+        break;
+
+      case 'getAllSubjects':
+        $subjects = Subjects::getAll();
+
+        if ($subjects === false)
+          self::returnError();
+
+        self::returnPayload([
+          'subjects' => $subjects
+        ]);
+        break;
+
+      case 'getUserSubjects':
+        self::checkSignInStatus();
+        // @TODO: Implement this method
+        break;
+
+      case 'setUserSubjects':
+        self::checkSignInStatus();
+        // @TODO: Implement this method
+        break;
+
+      case 'getClasses':
+        self::checkSignInStatus();
+        // @TODO: Implement this method
+        break;
+
+      case 'setClassState':
+        self::checkSignInStatus();
+        // @TODO: Handle this method
+        break;
+
+      default:
+        self::returnError('The method requested doesn\'t exist.');
+        break;
+    }
+  }
+}
diff --git a/inc/Auth.php b/inc/Auth.php
new file mode 100644
index 0000000..c0fc078
--- /dev/null
+++ b/inc/Auth.php
@@ -0,0 +1,59 @@
+<?php
+namespace DAFME\Covid;
+
+class Auth {
+  private $client;
+
+  public function __construct() {
+    global $conf;
+    $this->client = new \Google_Client();
+    $this->client->setApplicationName = 'dafme-covid-tracability-backend';
+    $this->client->setClientId($conf['goog']['clientId']);
+    $this->client->setClientSecret($conf['goog']['secret']);
+    $this->client->addScope('https://www.googleapis.com/auth/userinfo.email');
+    $this->client->setRedirectUri($conf['fullPath'].'oauth2callback.php');
+    $this->client->setAccessType('online');
+  }
+
+  public function getAuthUrl() {
+    return $this->client->createAuthUrl();
+  }
+
+  public function handleCallback() {
+    global $_GET, $con;
+    if (isset($_GET['error']) || !isset($_GET['code'])) return 1;
+
+    $accessToken = null;
+
+    try {
+      $accessToken = $this->client->fetchAccessTokenWithAuthCode($_GET['code']);
+    } catch (\Exception $exception) {
+      return 2;
+    }
+
+    $id = $this->client->verifyIdToken();
+    if ($id === false)
+      return 3;
+
+    if (!isset($id['sub']) || !isset($id['email']) || !isset($id['email_verified']))
+      return 4;
+
+    if ($id['email_verified'] === false)
+      return 5;
+
+    $sub = $id['sub'];
+    $email = $id['email'];
+
+    if (preg_match('/upc.edu$/', $id['email']) !== 1)
+      return 6;
+
+    if (!Users::signIn($sub, $email))
+      return 7;
+
+    return 0;
+  }
+
+  public function setAccessToken($token) {
+    $this->client->setAccessToken($token);
+  }
+}
diff --git a/inc/Subjects.php b/inc/Subjects.php
new file mode 100644
index 0000000..4d2cfdb
--- /dev/null
+++ b/inc/Subjects.php
@@ -0,0 +1,14 @@
+<?php
+namespace DAFME\Covid;
+
+class Subjects {
+  public static function getAll() {
+    global $con;
+    $query = $con->prepare('SELECT * FROM subjects');
+
+    if (!$query->execute())
+      return false;
+
+    return $query->fetchAll(\PDO::FETCH_ASSOC);
+  }
+}
diff --git a/inc/Users.php b/inc/Users.php
new file mode 100644
index 0000000..7679e04
--- /dev/null
+++ b/inc/Users.php
@@ -0,0 +1,51 @@
+<?php
+namespace DAFME\Covid;
+
+class Users {
+  private static function getUserId($sub) {
+    global $con;
+    $query = $con->prepare('SELECT id FROM users WHERE sub = ?');
+    if (!$query->execute([$sub]))
+      return false;
+
+    if ($query->rowCount() < 1)
+      return false;
+
+    $row = $query->fetch();
+    return $row['id'] ?? false;
+  }
+
+  public static function add($sub, $email) {
+    global $con;
+    $query = $con->prepare('INSERT INTO users (sub, email) VALUES (?, ?)');
+    if (!$query->execute([$sub, $email]))
+      return false;
+
+    return $con->lastInsertId();
+  }
+
+  public static function signIn($sub, $email) {
+    global $_SESSION;
+
+    $userId = self::getUserId($sub);
+
+    if ($userId === false)
+      $userId = self::add($sub, $email);
+
+    if ($userId === false)
+      return false;
+
+    $_SESSION['userId'] = $userId;
+    return true;
+  }
+
+  public static function signOut() {
+    global $_SESSION;
+    unset($_SESSION['userId']);
+  }
+
+  public static function isSignedIn() {
+    global $_SESSION;
+    return isset($_SESSION['userId']);
+  }
+}