This review was conducted to provide the reader with a brief overview of the science of epidemiology as applied specifically to physical activity as a behavior. Definitions were offered for epidemiology, for physical activity, and finally for physical activity epidemiology. In addition to considering epidemiologic concepts, this review stressed the importance of carefully measuring physical activity--perhaps the principal function of anyone who ultimately claims to be a physical activity epidemiologist. The breadth and scope of physical activity epidemiology are extensive and there is fertile ground for continuing to apply physical activity epidemiology to exercise science well into the future. This chapter should provide the exercise scientist with a firm foundation to begin his or her trek into the realm of physical activity epidemiology.