Authentication is a vital part of most web applications. Handling user registration and sign-in is an important feature, which can sometimes present a development overhead. Express, with its lean approach, lacks this feature, so, as is usual with node, an external module is needed. Passport is a Node.js module that uses the middleware design pattern to authenticate requests. It allows developers to offer various authentication methods using a mechanism called strategies, which allows you to implement a complex authentication layer while keeping your code clean and simple. Just as with any other Node.js module, before you can start using it in your application, you will first need to install it. The examples in this chapter will continue directly from those in previous chapters. So for this chapter, copy the final example from Chapter 5, Introduction to Mongoose, and let's start from there.