The Integrated Services Digital Network (ISDN) represents the current position in about a hundred years of evolutionary growth of the worldwide telecommunications infrastructure. This evolution is by no means complete and the next few years will see the emergence of a "Broad-band" ISDN as the next stage of evolutionary development. It is important to appreciate the evolutionary nature of the telecommunications infrastructure if one is to properly understand much of the thinking that lies behind the current ISDN proposals. This book therefore begins with a number of chapters devoted to a study of the various developments which have eventually led to the concept of an integrated digital network. These include the development of digital transmission of speech using PCM and the development of digital switching techniques based on stored program control. The book then turns to a consideration of those features of the existing telecommunications network which need to be modified in order to make ISDN a realizable practicality. Of particular importance is the digitization of transmission over the links between the user and the local exchange. Next we look at the current practice and proposals for ISDN based on the technology presently in use in the telephone network. Finally, we look at the proposals for a broadband ISDN likely to become widely available by the turn of the century.