Obesity is defined as an excess accumulation of body fat. To measure fat in the body accurately is difficult, and no method is easily available for routine clinical use. Traditionally, overweight and obesity have been evaluated by anthropometric measurement of weight-for-height. More recently, BMI has been used. The normal range is 19–24·9 kg/m2, overweight is 25–29·9 kg/m2, and obesity ≥ 30 kg/m2. Not only is the total amount of fat an individual carries important, but also where the fat is distributed in the body. Fat in a central or upper body (android) distribution is most related to health risk. The most accurate way to measure central obesity is by magnetic resonance imaging or computer-assisted tomography scanning, but this approach is too expensive for routine use. Simple anthropometric measurements can be used, such as waist circumference. A waist circumference of greater than 1020 mm in men and 880 mm in women is a risk factor for insulin resistance, diabetes mellitus and cardiovascular disease. There is a clear genetic predisposition for obesity. The genetic contribution to obesity is between 25 and 40 % of the individual differences in BMI. For the overwhelming majority of individuals, the genetic predisposition will not be defined by one gene, but by multiple genes. Eventually, classification of obesity may be done by genetic means, but this approach will require more knowledge.