The modern phenomenon of obesity is an archetypal example of a behavior whose explanation simultaneously falls within the purview of psychology, economics, and the biological sciences. While psychologists and advocates of public health have long viewed overeating as a weakness or disease in need of treatment, economists have pointed out that—like any other consumer behavior—choices about diet and exercise can be viewed from the perspective of rational decision theory, subject to the influence of variation in prices and income but not necessarily as a problem in need of a solution. Recent advances in our understanding of the physiological mechanisms by which genes influence behavior in modern socioeconomic environments have begun to point the way to a resolution to this debate. Drawing inspiration from the scientific literature on the neuroendocrinology of energy homeostasis, this paper reviews the empirical determinants of obesity in light of the biologist’s notion that humans and other animals evolved the ability to store body fat as an optimal response to the presence of starvation risk. This approach yields a powerful theoretical foundation, capturing such features of obesity as dynamic inconsistency, genetic variation, susceptibility to pharmaceutical intervention, and variation by season, socioeconomic status, and degree of financial security. It also provides a framework for reconciling the conflict between behavioral and neoclassical economics.