In this chapter, we explore key concepts and practical tools in relativistic kinematics. Topics include the transformation of four-vector between different reference frames, and the generation of initial- and final-state phase-space kinematics for decay and scattering events. We provide concrete examples demonstrating how to perform Lorentz boosts with ROOT’s built-in classes. A discussion is devoted to the two-body and three-body phase-space factors, including illustrative examples involving two-body scattering and three-body decay processes.