To the edition of 1980 have been added two new chapters, intended to convey the essence of recent developments in this ancient but ever- lively field. The text--addressed to graduate students and to advanced undergraduates--treats all the issues standard to its subject matter, but concentrates when possible on those aspects of the field which are at present "research active", and generally avoids complicated analytical machinery. Respectful attention is given to the history of the subject, and there are exercises (with hints) at the ends of the twenty chapters. Superbly produced.