This chapter taught us what a custom ROM is. We started from a description of the currently existing, most relevant projects, and we have proceeded deep into the details. We have also had a look at some very important components, such as Android Recovery, both the stock ones and those that have been modified. Finally, as we did in the previous chapters, we have adopted a practical approach, learning how to prepare a suitable environment for Android customization. We have also studied the different tools that are generally used to perform this task, and, in the end, we have applied the concepts we just learned through a simple example of creating a custom ROM. In the next chapter, we will be diving deeper into every single aspect of a ROM, using practical examples to show how to customize and increase the performance of your ROM.