Why study relativistic particle physics? Because of deeper understanding, curiosity and applications. Consider first deeper understanding. Physics forms the basis of many other sciences, and relativistic particle physics forms the basis of physics. Starting from nonrelativistic point mechanics, there are three major steps: first to classical (unquantized) relativistic electrodynamics, then to non­ relativistic quantum mechanics and finally to relativistic quantum physics. This book describes the third step. Relativistic particle problems which are mainly classical (such as synchrotron radiation) are largely omitted (see for example Jackson 1975). I have divided the subject into several smaller steps. The step from the Schrödinger equation to the Klein-Gordon and Dirac equations (chapter 1) is easy, apart from logical inconsistencies in limiting cases. Chapter 2 deals mainly with two-particle problems. From two-particle unitarity (sect. 2-5) and a symmetric treatment of projectile and target in the Born approxima­ tion to scattering (sect. 2-7), one is able to deduce recoil corrections to the relativistic one-particle equations (mainly the reduced mass, sect. 2-9). The final formulas provide a rather firm basis for atomic physics. Quantum electrodynamics (QED) is presented in chapter 3. Clearly, many things must be omitted if one allots one chapter to the subject of whole 1976, Källen 1958, Akhiezer and Berestetskii books (Jauch and Rohrlieh 1965, Bjorken and Drell 1965, Landau and Lifshitz 1971, 1975, and others).