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...