Objective The aim of this systematic review was to summarise and update the existing literature on determinants of physical activity and sedentary behaviour in young people, considering the methodological quality of the studies. Methods: Prospective studies were identifi ed from searches in PubMed and PsycINFO, from April 2004 through November 2010. The authors included studies investigating the association between determinants of overall physical (in)activity and/or sedentary behaviour in healthy children or adolescents. When a determinant was investigated for its association with physical (in) activity and sedentary behaviour assessed between ages of 4-12, or mean age ≤12, it was classifi ed as 'child determinant'. When a determinant was investigated for its association with physical activity and sedentary behaviour assessed between ages of 13-18 or mean age >12, it was classifi ed as 'adolescent determinant'. Included articles were scored on their methodological quality and a best-evidence synthesis was applied to summarise the results. Results: The authors identifi ed 30 papers, of which seven were of high methodological quality. Intention was found as a determinant of children's physical activity. Determinants of adolescents' physical activity were age (ie, older children were more active), ethnicity (ie, being of African-American descent determined being less physically active) and planning. The authors found insuffi cient evidence for determinants of sedentary behaviour. Conclusion: Studies on determinants of physical activity and sedentary behaviour were in general of poor methodological quality. To develop long-term effective interventions that increase physical activity and decrease time spent in sedentary behaviours in young people, we need more high quality prospective evidence on the determinants of these behaviours.