This chapter provides an overview of the main groups of methods and tools for XR development: programming languages and libraries, 3D content formats, and visual 3D and animation modeling tools, including game engines. The solutions are presented with the focus on behavior representation and modeling, which is relevant to the main contribution of this book. We also outline the main human–system interfaces for XR, including presentation and interaction devices, such as motion and eye tracking systems, controllers, and displays.