Obesity is a progressive disease of unwanted fat accumulation which has multiple, organ-specific pathological consequences. The manifestations of obesity occur within virtually every subspecialty of medicine or surgery and they interact importantly to accelerate the ageing process in many organs. Many of the hazards of obesity have multiple causes (e.g., diabetes, heart disease, stroke, colonic and breast cancer, urinary incontinence, tiredness, back pain, breathlessness). All of these conditions become more prevalent with age and are also more prevalent among overweight persons, particularly those with a central fat distribution marked by a high waist circumference. Hypertension may be caused or aggravated by weight gain. It is mediated by the physical demands of an expanded circulating volume and increased metabolic rate by metabolic mechanisms related to central fat distribution and the "metabolic syndrome", and to increased sodium consumption by overweight people (because they need more food to maintain a higher metabolic rate). Since body mass index (BMI) and waist circumference increase significantly with age there is an escalation of the burden of ill health from obesity with age. The best simple indicator of disease risk with obesity is the waist circumference since this identifies people who have a high body fat content and also those who have an increased intraabdominal accumulation of fat. The quantitative burden of ill health from overweight and obesity varies within different specialties, but up to 80% of type 2 diabetes or polycystic ovarian syndrome can be attributed to obesity. Obesity is the cause of sleep apnea syndrome in around 50% of cases and heart disease in perhaps 10-20% of cases. In Scotland 80% of people with existing cardiovascular disease are overweight compared with 57% of the general population. The financial burden to health services from overweight and obesity has been incompletely assessed, although it is estimated that around 4% of total health care budgets are attributable to people having BMI > 25 kg/m(2). This is similar to the entire cost of diabetes, epilepsy or major cancers. Obesity is therefore an extremely expensive disease based on these conservative estimates from limited evaluations. More general assessments show how obesity increases the amount of time taken off work, the number of drugs prescribed and the expenditure from social services support. Thus, obesity represents a huge burden not only on the individual patient physically, psychologically, socially and financially but also on families and careers and is a huge drain on health care resources. Overweight affects well over half of all adults worldwide, progressing to BMI > 30 kg/m(2) in around 20% outside subsistence rural communities. Its rapidly increasing prevalence now described as an epidemic demands major preventive measures, as well as better medical treatment for individuals affected.