This book grew-how could it be otherwise?-out of a series oflectures which the author held at the University of Heidelberg. The purpose ofthese lectures was to give an introduction to the phenomenology of elementary particles for students both of theoretical and experimental orientation. With the present book the author has set himself the same aim. The reader is assumed to be familiar with ordinary nonrelativistic quantum mechanics as presented, e.g., in the following books: Quantum Mechanics, by L.1. Schiff (McGraw-Hill, New York, 1955); Quantum Mechanics, Vol. I, by K. Gottfried (W.A. Benjamin, Reading, Ma., 1966). The setup of the present book is as follows. In the