This book is based on my lectures on elementary particles delivered during the fall semester of 1961-1962 at the University of Illinois. Some later additions were made in order to cope with recent developments in this field. The primary purpose of the book is to acquaint students with the historical development of elementary particle physics through exploration of the process of establishing a theory by comparison with experiment. In writing this book I have presupposed a basic knowledge of field theory, since it is the only adequate language to describe the properties of elementary particles. Nevertheless I hope that most of the subjects treated in this book—except for the latter half of Chapter 4, and Chapter 5-can be understood without this knowledge. I did not try to add an introduction to field theory because many textbooks on this subject are available. However, the emphasis has been placed on displaying detailed calculations that are often not explicitly given in original papers.