Excess weight and obesity are major public health challenges in this country Over 97 million adults are overweight or obese and it is rapidly reaching epidemic proportions among younger individuals. Overweight is defined as a body mass index (BMI) of 25 to 29.9 Kg/m2 and obesity as BMI of > or =30 Kg/m2. However, excess weight and obesity are not mutually exclusive since obese persons are also overweight. Obesity is a complex multifaceted condition that develops from the interaction of genetic, cultural, socioeconomic, behavioral, physiologic, metabolic, cellular, and molecular influences. Evidence is overwhelming to support the theory that obesity carries substantial economic, behavioral, physiologic, metabolic, cellula, and molecular influences. This article describes the major health risks that are associated with obesity and, to the extent known, the physiologic pathway of obesity for developing specific diseases. Clearly the evidence as presented has important implications for health promotion and disease prevention. It is also possible to consider the evidence and develop a theoretical basis for implementing sound behavioral and lifestyle interventions.