Discusses the relation between exercise and obesity in terms of etiology, physiology, and treatment outcome. It is considered that the role of activity level and caloric intake in the development of obesity is currently unclear because of the methodological limitations of past research. Salutary physiological effects of exercise on caloric intake, metabolic rate, and body composition are noted, and it is concluded that these changes may significantly affect energy output. Methodological problems with previous exercise treatment studies include failure to (a) use body composition measures as dependent variables, (b) report specifics regarding exercise tasks and participation rates, and (c) divide Ss into homogeneous populations based on qualitative aspects of body-fat type. Specific treatment guidelines, based primarily on the physiological data, are provided. (171 ref) (PsycINFO Database Record (c) 2016 APA, all rights reserved)