Multiple factors appear to influence and promote the development of obesity: The importance of genetic factors has been demonstrated in some studies, however, it is very difficult to distinguish between environmental and genetic influences. Independently, increased birthweight, massive weight gain in the first months after birth and overweight of the mother or of both parents seem to be risk factors, which are able to promote the development of childhood obesity. In the past few years more attention has been paid to psychological factors and their influence on appetite, physical activity and energy balance. Whether metabolic changes on cellular and microcellular basis can cause obesity is not sufficiently known until now. Nutrition in early childhood and fat cell hyperplasia and hypertrophia induced by nutritional factors probably do not imply persistence of obesity but may promote obesity and worsen the prognosis of therapy. Due to the fact that a high percentage of obese adolescents remain obese in adulthood, and due to the poor results in the treatment of obesity as well as the association of overweight with an increased risk for morbidity and mortality an effective therapy and prevention of obesity even during childhood seem to be of great importance.