The book presents some key mathematical tools for the performanceanalysis of communication networks and computer systems. Communication networks and computer systems have become extremelycomplex. The statistical resource sharing induced by the randombehavior of users and the underlying protocols and algorithms mayaffect Quality of Service. This book introduces the main results of queuing theory that areuseful for analyzing the performance of these systems. Thesemathematical tools are key to the development of robustdimensioning rules and engineering methods. A number of examplesillustrate their practical interest.