Abstract Obesity is increasing sharply around the world, in both adults and children. In the UK, national surveys show that obesity has trebled in 20 years and now affects almost a quarter of the adult population. There is a similar incidence of overweight in young people. Obesity is associated with significant increases in morbidity (especially type 2 diabetes) and the risk of premature mortality (especially from cardiovascular disease). Reductions in body weight of even 5–10% are associated with a significant reduction in the risk factors for chronic disease. The causes of obesity are multi-factorial. Rare cases of monogenic obesity have been described, but overall only about a third of the variance in weight can be ascribed to genetic factors. There is no evidence for substantial differences in metabolism between lean and obese subjects, indeed obese people have high energy needs. However, recent changes in diet and lifestyle, including low levels of physical activity and energy dense diets, are unmasking the genetic susceptibility to obesity and it is becoming increasingly difficult for the population at large to maintain energy balance. Changes in the nutritional composition of the core diet may be further compounded by larger portions, soft drinks and snacking, leading to an increased risk of weight gain. This chapter briefly reviews the epidemiology of obesity and examines the links to ill-health and premature death. It considers the causes of obesity including the role of genes, metabolism, diet, physical activity and the influence of the wider social and cultural environment which characterizes 21st century living.