Context: Social network analysis has been used to better understand the influence of friends and peer groups in a wide range of health behaviors. This systematic review synthesizes findings from various social network analyses of child and adolescent physical activity, to determine the extent to which social network structure is associated with physical activity behaviors. Evidence acquisition: Medical and social science databases were searched and screened between September and November 2011. Eligible studies collected a measure of physical activity and a measure of an individual's social network, either through friendship nominations or social ratings, and reported analyses investigating the association between physical activity and the social network measure. Evidence synthesis: A total of 1767 articles yielded nine publications from seven eligible studies, which were synthesized and analyzed in December 2011. Three research themes were identified: (1) friendship similarities in physical activity; (2) peer group influences on physical activity; and (3) social preference (i.e., popularity) and physical activity. Synthesis of findings across studies found strong evidence for similarities in physical activity levels between an individual and their friends and within peer groups. There was mixed evidence for an association between social preference and physical activity levels. Conclusions: Friendship plays an important role in shaping physical activity behaviors. Physical activity interventions targeted at peer groups and that account for the influence of friendship groups might have utility as a means of increasing youth physical activity.