These lectures throw a spotlight on different aspects of the evolution of telecommunications networks, namely on the various facets of service and network convergence. The last years progress in data and telecommunications technologies, such as IP-based networks, and the enormous potential of mobile communication systems and users' demands for comprehensive and network-independent have led to a convergence of data and telecommunications infrastructures in many aspects. In order to help the reader to an easier understanding of the phenomenon convergence, in the first two parts of this volume the evolution of the basic technologies is described one by one. This is done briefly and is focused on the principle topics, just to build a basis for the third part devoted to problems of convergence in telecommunications. These notes are addressed to those readers, who in a quick overview want to be informed on the future service and network landscape. The notes are equally suited for professionals with the desire to extend their horizon as well as for students looking for an introduction into telecommunications under more general aspects. The authors clearly understand the difficulties in writing a book devoted to the evolution in telecommunications. Today, telecom landscape varies at very high speed. Every few months new network technologies, new products and new services are developed. Attempts to present them in time can be accessible only for magazine publications or contributions to conferences. Therefore, in a number of areas, such as Voice over IP and new switching technologies not much more than the starting point of new paradigms is described. However, the content is up-to-date to a degree, that the phenomenon convergence can be fully understood. Partially, these notes are based on a number of lecture courses that were delivered during recent ICTP winter schools devoted to multimedia and digital communications. In order to secure full authenticity, a couple of experts have given their support to Section 3 (Convergence) of this volume. They were Jens Voigt (IP networks), Wolfgang Fleig (customer premises and corporate networks), Werner Kerschbaumer and Helmut Becker (fixed/mobile convergence), Helmut Muller (computer-telephony integration) and Tilman Durbeck (terminals). All of them are affiliated to Siemens AG, Munich. (ISBN 92-95003-16-0)