This book contains material that should interest students of electrical engineering and computer science specializing in digital communications and also practicing electrical engineers who apply digital communications techni­ ques to telecommunication systems, digital radio, digital satellites, fiber optics, and the physical layer of computer networks. This book is an outgrowth of lecture notes prepared over a number of years at various universities. In the earlier years I benefited immensely from t 5 the excellent textbooks and monographs in preparing my notes. - With passing time I had to rely more and more on the current periodical literature, mainly the IEEE Transactions and the Bell System Technical Journal. Although the book is intended mainly for those who have already had an introduction to communications, as usually taught in an undergraduate course, it can also be used without this background. For that purpose I concentrated most of the necessary mathematics in Chapter I. If the mathematics is not an obstacle, the reader can start with Chapter 2. I tried, as far as possible, to make each chapter independent of the other chapters, and for that reason many concepts and notations have been defined several times. To keep the book at a reasonable length, however, it was impossible, in most cases, not to rely on derivations and results of previous chapters.