The level of fatness at which morbidity increases is determined on an acturial basis. Direct measurements of body fat content, eg hydrodensitometry, bioimpedance or DEXA, are useful tools in scientific studies. However, body mass index (BMI) is easy to calculate and is frequently used to define obesity clinically. An increased risk of death from cardiovascular disease in adults has been found in subjects whose BMI had been greater than the 75th percentile as adolescents. Childhood obesity seems to increase 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 on the role of neuropeptides such as POMC, neuropeptide Y (NPY) and the melanocyte concentrating hormone receptors (MC4R). Environmental/exogenous factors contribute to the development of a high degree of body fatness early in life. Twin studies suggest that approximately 50% of the tendency toward obesity is inherited. There are numerous disorders including a number of endocrine disorders (Cushing's syndrome, hypothyroidism, etc) and genetic syndromes (Prader-Labhard-Willi syndrome, Bardet-Biedl syndrome etc) that can present with obesity. A simple diagnostic algorithm allows for the differentiation between primary or secondary obesity. Among the most common sequelae of primary childhood obesity are hypertension, dyslipidemia and psychosocial problems. Therapeutic strategies include psychological and family therapy, lifestyle/behavior modification and nutrition education. The role of regular exercise and exercise programs is emphasized. Surgical procedures and drugs used as treatments for adult obesity are still not recommended for children and adolescents with obesity. As obesity is the most common chronic disorder in the industrialized societies, its impact on individual lives as well as on health economics has to be recognized more widely. This review is aimed towards defining the clinical problem of childhood obesity on the basis of current knowledge and towards outlining future research areas in the field of energy homoeostasis and food intake control.