It is a nontrivial problem to generalize quantum mechanics to describe relativistic particles, and as we shall see, the solution contains some rather unusual features. First we shall study spinless, or scalar, particles and then later particles with spin. The reason is that the properties of spin, an angular momentum, are closely subject to the requirements of special relativity and as a consequence the form of the wave equation for a particle depends critically on its spin. Common relativistic spinless particles are, for example, π and K mesons.