Authentication is one of the most common features of web applications, so it makes sense to have libraries that provide solutions for this problem. You've probably heard of or maybe used Devise: all you have to do is add it to your Gemfile and run a generator, and you have a robust authentication system. Behind the scenes, Devise uses Warden to handle authentication. In this talk, I'll explain what Warden is, why it's useful and how Devise takes advantage of it to build the most popular authentication gem for Rails.