A Crystal method is a lightweight method with characteristics such as emphasis on people instead of processes and products, fast communication (preferably by working together in one room), the quick delivery of products, frequent and automatic testing, and regular evaluations.

Unlike some other software development processes, Crystal is not a software development method, but a collection of methods and processes. This collection is called the Crystal Family. Crystal was invented and described by Alistair Cockburn. Each member of the Crystal Family is indicated with a color representing the weight of a method, where the following applies: the darker the color, the heavier the method. The color of the method is chosen on the basis of the size and severity of the project. The size is determined by the number of people participating in the project, and the severity is determined by the risk that choosing the method could cause systemic damage. The colors are, like real crystals, sorted from light to dark. Crystal clear is the smallest and lightest, followed by yellow, orange, orange web, red, maroon, blue, violet, and so on.

Although the Crystal methods differ from each other, they do have some similarities, which is why they are also called a family. They have three common priorities: safety, efficiency, and usability. Furthermore, they also have common characteristics, the three most important of which are the frequent delivery of (intermediate) products, feedback on improvements, and good communication.

We have already discussed a team-based approach to product development with certain process steps, and now with Crystal methods, a lightweight Agile method that focuses on people within the team itself. There are also more radical methods that focus not only on people and process, but also on tooling and technical quality. We will discuss these XP methods in the following section.