This systematic review investigated the relationship between peer and/or friend variables and physical activity among adolescents by synthesising cross-sectional, longitudinal, and experimental research conducted in the US. Seven electronic databases were searched to identify related articles published within the last 10 years and the articles reviewed included adolescents between 10 and 18 years. Studies reporting a measure of physical activity for adolescents and at least one potential peer and/or friend variable were included. Research demonstrated that peers and friends have an important role to play in the physical activity behavior of adolescents. Six processes were identified through which peers and/or friends may have an influence on physical activity including: peer and/or friend support, presence of peers and friends, peer norms, friendship quality and acceptance, peer crowds, and peer victimization. The theoretical significance of these results is assessed and the development of peer-related physical activity programs for adolescents is discussed. (Contains 1 table.)