The industrialized countries around the world are experiencing an epidemic of childhood obesity. The level of fatness of a child at which morbidity increases acutely and/or later in life is determined on an individual basis. Overall, however, childhood obesity substantially increases the risk of subsequent morbidity whether or not obesity persists into adulthood. The genetic basis of childhood obesity has been elucidated to some extent through the discovery of leptin, the ob gene product, and the increasing knowledge of the role of neuropeptides such as pro-opiomelanocortin, neuropeptide Y and the melanocyte-concentrating hormone receptors. Environmental and exogenous factors are the main contributors to the development of a high degree of body fatness early in life. Studies involving twins suggest that approximately 50% of the tendency toward obesity is inherited. There are numerous disorders, including a number of endocrine disorders, such as Cushing's syndrome and hypothyroidism, and genetic syndromes, such as Prader-Labhard-Willi syndrome and Bardet-Biedl syndrome, that can present with obesity. A simple diagnostic algorithm allows for differentiation between primary and secondary obesity. Among the most common sequelae of primary childhood obesity are hypertension, dyslipidemia, back pain and psychosocial problems. It is somewhat ironic that the definition of obesity in childhood is not an easy one. Direct measurements of body fat content, such as hydrodensitometry, bioimpedance, or dual-energy X-ray absorptiometry, are useful tools in scientific studies. Body mass index (BMI) is, however, now generally accepted to be a good clinical measure for the definition of obesity in children and adolescents. In preadolescent boys, BMI also relates to muscle mass and should be used for the definition of fat mass with great caution. An increased risk of death from cardiovascular disease in adults has been found in patients whose BMI had been greater than the 75th percentile as adolescents. Therapeutic strategies include psychological and family therapy, modification of lifestyle and behavior, and nutritional education. The role of regular exercise and exercise programs is emphasized, while surgical procedures and drugs used in adult obesity are still not generally recommended for obese children. Obesity is the most common chronic disorder in industrialized countries, and its impact on individual lives as well as on health economics must be recognized by physicians and the public alike. This review aims to increase awareness of the health burden and economic dimension of the epidemic of childhood obesity that is occurring around the globe.