The aim of this review is to provide a scientific update on evidence related to the biological and psycho-social health benefits of physical activity in school-aged children and youth. To accomplish this aim, the first part of the paper reviews existing physical activity guidelines for school-aged children and youth, with an emphasis placed on how Canada's guidelines compare and contrast with those of other countries and organizations. The paper then provides an overview of physical activity levels of Canadian children and youth, which indicates that few Canadian youngsters meet current physical activity recommendations. The next section of the paper summarizes the literature that informs how much physical activity is required to promote health and well-being in children and youth. The paper then provides suggestions on modifications that could be made to Canada's physical activity guidelines for children and youth. Specifically, consideration should be given to setting both minimal (> or =60 min/d) and optimal (up to several hours per day) physical activity targets. The final section identifies future research needs. In this section, a need is noted for comprehensive dose-response studies of physical activity and health in the paediatric age group.