This paper analyzes the forces contributing to the worldwide long-run rise in obesity and the role of public interventions in affecting its continued growth. A growth in obesity in a population must result from the growth of calorie consumption outpacing the growth of physical activity. Yet historically in developed countries, obesity has grown with modest rises in calorie consumption and with a substantial increase in both exercise and dieting. We consider the economic incentives that give rise to the long-run growth in obesity by stimulating intake of calories at the same time as discouraging the expending of calories on physical activity, whether in work or leisure. We argue that technological change provides a natural interpretation of the long-run growth in obesity, that it predicts that the effect of income on obesity changes from positive to negative with economic development, and that it implies that technological change may not continue to raise weight. We discuss the positive and normative impacts of direct and indirect public interventions to reduce obesity.