Publisher Summary The theories of special relativity and general relativity together represent one of mankind's greatest intellectual achievements. This chapter presents the development or measurement of the theory of special relativity in constant-velocity reference frames. Special relativity has had a major impact on the field of physics, in the calculation and understanding of high-velocity phenomena. It has also had consequences that go beyond physics. The chapter presents special relativity as a generalization of Galilean relativity, a generalization that is required for speeds comparable to the speed of light. The chapter introduces the 20th-century physics. Special relativity enables one to study dynamics from a 20th-century viewpoint. Special relativity is one of the most fascinating topics in physics. It involves re-educating one's common sense to bring it into agreement with experimental facts.