This book is intended for students who have had a course in atomic theory in addition to the usual introductory physics courses. Since it is often impossible to give separate instruction to students intending to continue the study of nuclear science and to those intending to specialize in some other field, an attempt has been made to write a book suitable for both groups. To make it easy to follow the main thread of the discussion, the presentation in the text has been kept as simple as possible. Some of the problems and exercises at the ends of the chapters have been chosen with the intention of interesting students who may wish to think somewhat more deeply about matters discussed in the text.