This book is an introduction to analytical performance modeling for computer systems, i.e., writing equations to describe their performance behavior. It is accessible to readers who have taken college-level courses in calculus and probability, networking, and operating systems. This is not a training manual for becoming an expert performance analyst. Rather, the objective is to help the reader construct simple models for analyzing and understanding the systems in which they are interested. Describing a complicated system abstractly with mathematical equations requires a careful choice of assumptions and approximations. These assumptions and approximations make the model tractable, but they must not remove essential characteristics of the system, nor introduce spurious properties. To help the reader understand the choices and their implications, this book discusses the analytical models in 20 research papers. These papers cover a broad range of topics: processors and disks, databasesand multimedia, worms and wireless, etc. An Appendix provides some questions for readers to exercise their understanding of the models in these papers. Table of Contents: Preliminaries / Concepts and Littles Law / Single Queues / Open Systems / Markov Chains / Closed Systems / Bottlenecks and Flow Equivalence / Deterministic Approximations / Transient Analysis / Experimental Validation and Analysis / Analysis with an Analytical Model