The aim of this book is the development of the heavy traffic approach to the modeling and analysis of queueing networks, both controlled and uncontrolled, and many applications to computer, communications, and manufacturing systems. The methods exploit the multiscale structure of the physical problem to get approximating models that have the form of reflected diffusion processes, either controlled or uncontrolled. These ap- proximating models have the basic structure of the original problem, but are significantly simpler. Much of inessential detail is eliminated (or "av- eraged out"). They greatly simplify analysis, design, and optimization and yield good approximations to problems that would otherwise be intractable, under broad conditions. Queueing-type processes are ubiquitous occurrences in operations re- search, and in communications and computer systems. Indeed, it is hard to avoid them in modern technology. The subject is now about 100 years old. and there is an enormous literature. Impressive techniques, many based on Markov chain and ergodic theory, have been developed to han- dle a great variety of models. A sampling of the numerous books includes [6, 8, 18, 27, 33, 46, 81, 86, 132, 133, 220, 243]. But the models of interest are growing fast in the face of the demands of new applications, particularly in communications and computer systems.