In this chapter we introduce the relativistic expressions for the momentum and energy of a particle. The reader should note that the adjective relativistic that we have just used has come to mean a qualification that pertains to Einstein's special relativity or general relativity although before Einstein, a principle of relativity was introduced by Galileo (Section 5.1). Thus a relativistic speed is one that is a significant fraction of the speed of light. And what do we mean by particle? Strictly it is an object with mass but without size or shape, sometimes called a point mass (Section 5.2). In Newtonian mechanics many bodies that are extended may be approximated by particles. In modern physics the word particle is applied to the atomic and sub-atomic constituents of matter (Section 3.2).