Since it is generally accepted that the onset of many chronic diseases lies in early childhood, preventive strategies should start as early in life as possible. Physical inactivity is recognised as an important determinant for chronic disease and the prevalence of physical inactivity is increasing during adolescence. Therefore, in the last 10 years much effort has been put into the development of physical activity guidelines for children and adolescents. In this critical review it is shown that the scientific evidence on which these guidelines are based is rather weak. There is only marginal evidence that physical activity during youth is related to adult health status. There is, however, hardly any evidence for a certain dose-response relationship or a particular threshold value from which guidelines can be obtained. Furthermore, it can be suggested that although the value of physical activity guidelines for children and adolescents for public health purposes is beyond doubt, perhaps these guidelines should focus on aspects other than possible health benefits.