The recent unexpected increases in the prevalence of obesity in the United States are widely agreed to be the result of changes in environmental conditions. This paper reviews the available data from diverse sources on environmental factors and obesity. Coverage includes descriptive data on temporal trends in the environment, cross-sectional and longitudinal studies of the association between environmental exposures and body weight, and experimental trials that have related environmental factors thought to be potentially important in influencing energy intake and expenditure and body weight. Over the period covered by the "obesity epidemic," a variety of environmental factors have changed dramatically. Some would seem to favor increased body weight (e.g., increased availability of convenience foods and increased use of automobiles and televised entertainment), and others would seem to favor decreased body weight (e.g., a lower-fat food supply and the increased availability of some forms of physical activity). Definitive conclusions about the relative contributions of energy intake and expenditure to increasing body weight or about the contribution of specific environmental exposures to increasing body weight are far from clear. Increased sophistication in methods for making valid inferences from existing environmental data would be helpful. Even more important, given the urgency of the problem, is experimental research on the question of what environmental changes would be necessary to reverse the obesity epidemic.