Elizabeth Duffy argued that physiological arousal, or activation, is more basic to behavior and performance than is the learning process. It has long been recognized that physiological arousal is a basic and important factor in health and behavior. Stress has been extensively studied with regard to the increases in arousal that it produces and the adverse effects on health that it can have. In addition, a substantial literature identifies and studies drugs that contribute to arousal. In particular, these include caffeine, which is arguably the single most common source of increments in arousal. Heightened arousal has widespread physiological and behavioral consequences. Increments in arousal are associated with elevations in anxiety, hostility, and irritability; chronically high arousal may contribute to anxiety, somatoform, and depressive disorders, among others. The chapter also presents an overview of the key concepts discussed in this book.