Einstein published his ideas on relativity in two major instalments. The ﬁrst instalment 1905 examined how the perception of time, space and energy might diﬀer for observers in uniform relative motion. This constituted the basis of the special theory of relativity. When the speed of any particle is very high, close to the speed of light, the kinematical consequences of this theory were found to be drastically diﬀerent from those derived from Newton’s laws of motion and associated ideas of space and time. Since the study of particle physics is intimately connected to high energies and therefore large speeds, we must take relativistic eﬀects into account in discussing kinematics of particle motion, which is what we do in this chapter. We will be brief, because we will assume that the reader is familiar with the basic tenets of the special theory of relativity. For fuller treatments, the reader is advised to consult textbooks on the subject.