Probabilistic modeling is the most cost-effective means of performance evaluation of computer systems. This book is a major revision of Modelling of Computer Communication Systems (CUP, 1987), one of the standard introductions to the area. Changes to the content reflect the change in the subject itself. Professor Mitrani has amplified the treatment of queues, reliability and applied probability. This book will be welcomed by students and teachers for its no-nonsense treatment of the basic results and examples of their application. The text includes the necessary fundamentals in probability and stochastic processes, making the book ideal for students in computer science or operations research taking courses in modern system design.