The principles of classical mechanics that we studied in the early chapters of this text give us an appropriate way to describe the motion of objects with a normal size moving at normal speeds. Also, the basic principles of electromagnetism and electromagnetic waves were summarized in Maxwell’s Equations and they did a good job of explaining this area of classical physics. However, in the early part of the twentieth century physicists began studying atomic size particles and objects traveling at speeds close to the speed of light. At these extreme conditions they found that normal classical physics could not explain all of their observations. This spawned the birth of Modern Physics based on quantum mechanics and relativity. After discussing the basic ideas of these theories in this chapter, we will see how they are applied to atomic and nuclear physics in the final chapter. Some of the themes of the chapter are represented in Fig. 15.1.