Obesity is a state of excess adipose tissue mass. Obesity should not be defined by body weight alone, as muscular individuals may be overweight by arbitrary standards without having increased adiposity. The most widely used method to classify weight status and risk of disease is the body mass index (BMI), which is equal to weight/height² in kg/m² (Table 172-1). At a similar BMI, women have more body fat than men. Furthermore, regional fat distribution may influence the risks associated with obesity. Central (primarily visceral) obesity (high ratio of the circumference of the waist to the circumference of the hips [waist-to-hip ratio], >0.9 in women and 1.0 in men) is independently associated with a higher risk for metabolic syndrome, diabetes mellitus, hyperandrogenism in women, and cardiovascular disease. The prevalence of obesity has increased dramatically over the past 3 decades. In the United States, about 34% of adults age >20 are obese (BMI >30), and another 34% are overweight (BMI 25-30). Most alarming is a similar trend among children, where about 16% of adolescents are obese. This has led to an epidemic of type 2 diabetes in children, a condition almost never seen until recently. These trends to increased obesity are not limited to Western societies but are occurring worldwide.