SDL (the Specification and Description Language for Telecommunication Systems) has evolved into a fully-fledged tool for the specification of telecommunication (sub-)systems, and is proving its fitness for major applications. The current (1988) version, the subject of this book, is used in the specification of the Functional Reference Model for Integrated Broadband Communication being studied in RACE, the European Community R&D programme in Telecommunications. For the many potential users of SDL, whether in industry or in the operating companies, this book should be of great practical value. By using worked examples from actual telecommunications engineering practice, and by explaining conceptual choices as they are made, it will enable students to learn the many features of the language in coherent combinations, and thus inspire correct usage from an engineering point of view. I congratulate the authors on their initiative and wish their readers success in mastering this invaluable addition to the tools-of-the-trade. From the preface by Rudolf W. Meijer, Commission of the European Communities Directorate General Telecommunications, Information Industries and Innovation.