Obesity prevalence has doubled among adults and overweight has tripled among children since 1980. This article discusses behavioral approaches to the obesity epidemic, focusing on recent environmental changes, the resulting behaviors, and possible solutions. Over the last 4 decades, time spent in sedentary activities, the consumption of fast food, the structural environment, and the percentage of women in the work force have changed dramatically. The cumulative effect of these changes on the eating and exercise habits adopted by Americans has been disastrous. In order to reverse the obesity trend, physical and social environments that support healthful habits must develop. Behavioral approaches to weight loss have proven to be the most effective at promoting individual health behavior changes; however, these changes will be much more likely to persist if supported by family, school, and work environments, organizations, policies, and communities. (Contains 1 figure.)