Obesity is now the most frequent nutritional disorder of childhood in the so‐called developed countries of the world. It is due to an increase in the amount of adipose tissue but, because it is technically difficult to measure the exact amount of adipose tissue, obesity is usually defined in terms of body weight. In general an individual who weighs 20% or more than the weight expected for his height and age can be regarded as being obese. By this criterion the incidence of obesity amongst schoolchildren in Britain is probably at least 3%, although accurate contemporary figures are not available. Studies in the United States of America have suggested that amongst adolescent girls between 10–15% can be expected to be overweight.