For the past five years, my editor at Springer-Verlag has asked me to write a second edition of this text that would incorporate new material on the quark model. Because this is a subject at the forefront of modern physics, whose central ideas are perpetually in flux, such an addition is not a simple task. Nevertheless, I have tried to discuss quark model topics that should stand the test of time and be of interest to introductory advanced quantum mechanics students as examples of the Feynman diagram technique. I have also tried to eliminate errors made in the first edition. I appreciate the work of R. Miller, who graciously typed the additional material. My colleagues V. Elias, T. Hakioglu, S. Kocic, N. Paver, and R. Thews helped me formulate the quark model chapter. Tucson, Arizona M. D. Scadron May 1990 vii Preface to the First Edition The fundamental goal of physics is an understanding of the forces of nature in their simplest and most general terms. Yet the scientific method inadver tently steers us away from that course by requiring an ever finer subdivision of the problem into constituent components, so that the overall objective is often obscured, even to the experts. The situation is most frustrating and acute for today's graduate students, who must try to absorb as much general knowledge as is possible and also try to digest only a small fraction of the ever increasing morass of observational data or detailed theories to write a dissertation.