Introduction: Physical inactivity has been identified as a serious problem and a major public health concern for people of all ages. More than a decade ago physical activity was labeled as “today’s best buy in public health” and WHO specifically identified schools as a target setting for the promotion of physical activity among children and youth. Aim: The overall aim of the study is to investigate the short and long term health effects of 4 addi¬tional lessons of physical education per week, compared to 2 lessons of physical education. The present five PhD-projects outlining the project are presented: •Physical activity and risk factors for lifestyle diseases in Danish school children •Bone health and physical activity in healthy Danish school children •Impact of increased amount and quality of physical education on physical performance and sports injuries in Danish school children •Impact of increased physical activity on neck and back pain in Danish school children. •Impact of increased physical activity on sports and leisure time related injuries in Danish school children Methods: A three-year controlled intervention study (2008-11) in 10 public schools. 1520 children aged 6 to 11 were invited to participate, 1220 accepted. Six intervention schools (689 children) and four control schools (531 children). Preliminary analysis shows no differences regarding an¬thropometrics, motor performance and aerobe fitness between intervention and control schools at baseline. Perspectives: The pragmatic design of this study makes it a virtual implementation study - easy to implement in other public schools in Denmark, and it provides: •A large cohort for future follow-up studies. •A tool for identifying ”specific target groups” and provides new information of dose-re¬sponse for health related physical activity. •A new insight regarding the relationship of vitamin D, vitamin K, physical activity, bone mineral density and the risk of fractures. •New knowledge on how to detect sports talent in childhood and adolescence. •New knowledge regarding sports and leisure-time related injuries, neck and back pain in children and adolescents, with the hope to enable development of novel prevention and intervention strategies.