This book describes the basic theory of performance engineering (essentially the control of traffic in communications, but nowadays applied to other aspects of systems design as well) and its application to both circuit- and packet-switched systems. As an increasing number of systems have become too complex to be analysed theoretically, simulation techniques are introduced. Also covered are other applications such as reliability and tolerances, including the relationship between radio fading and teletraffic in cellular mobile communications. The book is aimed at senior undergraduates as well as graduate and professional engineers and their managers.