The SOPHYA study investigated how children and adolescents from all over Switzerland move or engage in sports and how this behaviour changes at the transition from childhood to adolescence and from adolescence to adulthood. SOPHYA is the first nationwide long-term study that objectively measured the physical activity behaviour of children and adolescents (using accelerometers) and at the same time investigated the influencing factors by means of surveys on sports activities, family, lifestyle, living environment and health. This made it possible to identify short- and long-term factors influencing physical activity and sport behaviour.