This paper reviews the published cost-of-illness studies on obesity. The medical literature has demonstrated that obesity is an independent risk factor for a number of medical conditions, including diabetes mellitus, hypertension, coronary heart disease, elevated cholesterol levels, depression, musculoskeletal disorders, gallbladder disease, and several cancers. Since these conditions can be costly to treat, obesity clearly has a substantial economic impact. Epidemiologic estimates of the aggregate economic costs associated with specific obesity-related diseases in the United States indicate that the annual burden to society totals in the billions of dollars, representing 5.5% to 7.8% of total health-care expenditures. Although estimates of the costs attributable to obesity differ across studies, the one common finding is that these costs are substantial from a health-policy perspective. The objective of this paper is to identify and review the obesity cost-of-illness literature, address study limitations, and identify key areas for future economic research. This review indicates that the economic burden of obesity has been estimated using a prevalence-based cost-of-illness framework. Areas for future research include estimating the economic burden of obesity using an incidence-based cost-of-illness framework and modeling the association between health-care expenditure and level of obesity using individual-level data, such as medical and pharmacy claims data.