Body weight is regulated by a feedback loop in which peripheral signals report nutritional information to an integratory center in the brain. The cloning of the ob gene is consistent with this concept and suggests that body fat content in adult rodents is regulated by a negative feedback loop centered in the hypothalamus. In recent years a number of additional signaling molecules secreted by adipose tissue have been discovered. These hormones, named adipocytokines, include resistin, adiponectin, and visfatin. Among the adipocytokines, adiponectin is perhaps the most interesting compound for the clinician since low adiponectin serum levels have been found in obese subjects and in particular in insulin resistant subjects. The definition and diagnosis of obesity in children and adolescents are surprisingly difficult. The level of fatness at which morbidity increases is determined on an acturial basis. In children and adolescents the degree of body fat mass depends upon ethnic background , gender, developmental stage, and age. Treatment and prevention of obesity in childhood and adolescence are major challenges for today's health care providers and societies.