The authors examine the broad field of behavior therapy, the theories underlying its development, and the techniques that are used to change behavior. They review recent developments in behavior therapy as they relate to behavioral medicine and to treatment of psychiatric disorders, and briefly describe the use of cognitive behavior therapy in the treatment of depression and other problems. They discuss methods of behavioral assessment and the application of behavior therapy in large populations, as well as new uses of behavioral techniques in business and industry, gerontology, and marital therapy.