In the development of telecommunication networks throughout the world, digital transmission has now replaced analog transmission as the predominant choice for new transmission facilities. This trend began in the early 1960s when the American Telephone and Telegraph Company first introduced pulse code modulation as a means of increasing capacity in their cable plant. Since that time, digital transmission applications have grown dramatically, notably in the United States, Canada, Japan, and Western Europe. With the rapidity of digital transmission developments and imple- mentation, however, there has been a surprising lack of textbooks written on the subject. This book grew out of my work, research, and teaching in digital transmission systems. My objective is to provide an overview of the subject. To accomplish this end, theory has been blended with practice in order to illustrate how one applies theoretical principles to actual design and imple- mentation. The book has abundant design examples and references to actual systems. These examples have been drawn from common carriers, manufac- turers, and my own experience. Considerable effort has been made to include up-to-date standards, such as those published by the CCITT and CCIR, and to interpret their recommendations in the context of present-day digital transmission systems.