This book was written twenty years ago, but we are leaving it unchanged - not because little has happened to the subject, but because so much has happened that any change would require very major rewriting. On the other hand, no other book on the subject seems to have appeared and we hope this paperback edition will be of some use to a younger generation - if not for another twenty years, at least until someone else has the patience to write a new book. Although we have added no new material we have corrected some misprints. We have added new references only where they correct actual errors. We are grateful to Drs. G. Feinberg, M. Inokuti, Y. Kim, and A. Petermann for pointing out errors. Nearly all of this book is taken from an article prepared for a volume of the Encyclopedia of Physics. This article, in turn, is partly based on Dr. Norbert Rosenzweig's translation of an older article on the same subject, written by one of us (H.A.B.) about 25 years ago for the Geiger-Scheel Handbuch der Physik. To the article written last year we have added some Addenda and Errata. These Addenda and Errata refer back to some of the 79 sections of the main text and contain some misprint corrections, additional references and some notes.