The United States is in the midst of an epidemic of obesity involving more than one third of the adult population. The prevalence of obesity increased by 40% between 1980 and 1990. Obesity is a chronic disease with a multifactorial etiology including genetics, environment, metabolism, lifestyle, and behavioral components. A chronic disease treatment model involving both lifestyle interventions and, when appropriate, additional medical therapies delivered by an interdisciplinary team including physicians, dietitians, exercise specialists, and behavior therapists offers the best chance for effective obesity treatment. Lifestyle factors such as proper nutrition, regular physical activity, and changes in eating behaviors should be coordinated by this team. This review addresses the modern epidemic of obesity, the strong association between obesity and comorbidities such as coronary heart disease, type 2 diabetes, hypertension, and dyslipidemia. In addition to obesity, the health risks of abdominal obesity and adult weight gain are discussed. The evidence that supports health benefits from modest weight loss (between 5% and 10% of body weight) is evaluated and the 5 key principles of effective obesity therapy are put forward. Obesity is a therapeutic challenge best met by teams of health care professionals, including dietitians and physicians, working together to deliver optimal treatment.