The rapidly rising prevalence of obesity, worldwide, has prompted re-evaluations of the definitions and diagnostic criteria, and of the extent of the burden it contributes to health care services. Although categorized arbitrarily for epidemiological purposes according to BMI > 25 kg/m2 (‘overweight’) and BMI > 30 kg/m2 (‘obese’), the disease itself (ICD code E.66) is the process of excess fat accumulation. It leads to multiple organ-specific pathological consequences, particularly if there is a tendency to intra-abdominal fat accumulation. The simplest field method to identify obesity and risk of medical problems is the waist circumference, and this method has found a special role in health promotion. Risks begin with waist > 80 cm (women) or > 94 cm (men). As a broad generalization, obesity produces few symptoms below the age of 40 years, but then several symptoms often develop; tiredness, breathlessness, back pain, arthritis, sweatiness, poor sleeping, depression and menstrual disorders all being common. The symptoms are often attributed to diseases in other body systems. Metabolic diseases like diabetes, hyperlipidaemia and, hypertension develop later, but the mean BMI at diagnosis of diabetes is 28 kg/m2. Ultimately, obesity increases the likelihood of myocardial infarction, stroke and several major cancers, but its biggest impact on health, especially in the elderly, is probably the multiplicity of effects on other body systems. The greatest challenge for public health is to develop effective preventive measures, recognizing that BMI > 25 kg/m2 before the age of 20 years is a very strong predictor of obesity and ill health in adulthood.