This book presents revised versions of tutorial lectures given at the IEEE/CS Symposium on modeling, analysis, and simulation of computer and telecommunication systems held in Orlando, FL, USA in October 2003.The lectures are grouped into three parts on performance and QoS of modern wired and wireless networks, current advances in performance modeling and simulation, and other specific applications of these methodologies.This tutorial book is targeted to both practitioners and researchers. The practitioner will benefit from numerous pointers to performance and QoS issues; the pedagogical style and plenty of references will be of great use in solving practical problems. The researcher and advanced student are offered a representative set of topics not only for their research value but also for their novelty and use in identifying areas of active research.