--- title: Where to start? description: This document explains zitadels key capabilities --- ### I already have an authentication system If your app already has a sign-in implementation and you want to use it to authenticate with Zitadel backend services, read our `migration documentation`. ### I want to build my authentication system with Zitadel If you're building a new app or adding sign-in to an existing app, Zitadel has libraries and services that can help you implement secure authentication without having to build the authentication backend yourself. Zitadel Authentication is a complete backend solution for signing in with passwords and additional Factors for enhanced Security.