The prevalence and incidence of overweight or even obese children and adolescents is significantly on the increase worldwide. According to the German Children and Adolescent Health Survey (KIGGS) conducted in 2006, 15% of all children and adolescents in Germany aged 3 through 17 years are overweight, and 6.3% of these children and adolescents are obese. On account of the long-term consequences, such as the metabolic syndrome, it can be expected that this "crisis in public health" will lead to a significantly higher expenditure of economic resources in the health care sector. Therefore it is important that public health prevention strategies analyse the key causes of overweight and obesity, and that they not only incorporate individual behaviours regarding nutrition and physical activity, but also take environmental factors, such as the residential area and traffic situation, as well as political circumstances regarding the nutrition and social aspects, into account. Even though the definition of body-mass index (BMI) has been widely accepted, the epidemiological data and the drawn percentiles are in need of a solid interpretation. There are several causes for the rising prevalence in overweight and obesity which are currently being focused on and discussed. On the individual level, the focus is on the genetic disposition and the changes in the behaviour regarding nutrition and physical exercise. Additional key influential factors like the increase in urbanisation and motorisation, the respective changes in the living environment of children and their families, and migration with its specific biosocial and cultural implications are discussed from the environmental and sociomedical, as well as the public health perspective. The article concludes with a discussion on the consequences of effective prevention strategies with reference to the Cochrane analysis from 2005. In order to be effective and successful, interventions for the prevention of overweight and obesity have to look at the structures and the environment of the person, as well as at the behavioural aspects of the individual. Such elements will be outlined based on the German "Platform Physical Activity and Nutrition".