Childhood obesity influences body weight in the adult and is intrinsically associated with multiple co-morbidities. In the past 20 years, the prevalence of overweight and obese school children in Switzerland has increased by three to six-fold. One out of every four to five children is overweight or obese. Thus, primary prevention is absolutely essential. Excess weight results from a positive energy balance. According to the current literature, changes in the quantity and quality of nutrition, a reduction in physical activity, an increase in sedentary lifestyles, including media consumption, as well as a reduction of sleep time are the most important external factors that promote the development of childhood obesity after infancy. Hereby, the intrauterine milieu and genetic factors also play a role. The obesity epidemic particularly affects children born to overweight parents, children with low socio-economic status, and migrants. Randomized, controlled studies aimed at medium to long-term (> or =1 year) reductions in BMI or fat tissue have mainly been school-based and sometimes involved the family as well. Unfortunately, these studies only produced unanimously negative or modest results. We believe that successful and sustainable prevention must contain 3 elements: (1) A relatively intensive and sustained modification of the individual's behavior that appropriately accounts for the multifactorial causes of childhood obesity. (2) A concurrent adaptation of external conditions that enable behavioral modifications. (3) Incorporation of socio-economic and political aspects.