Physical activity is important for health promotion and disease prevention. Effective physical activity programs for adolescents require a proper understanding of the determinants of activity levels. The main purpose of this paper was to review the scientific literature on determinants of physical activity among adolescents: demographic, biological (age, gender, socioeconomic status), and socio-cultural (family, peers, and physical education teachers). The review included only studies with large samples (>100 subjects) and a cross-sectional design, and that used questionnaires to measure physical activity in adolescents (10-18 years). The main results and conclusions were: age is negatively associated with physical activity; boys tend to be more active than girls; higher socioeconomic status is positively associated with more physical activity; adolescents are more involved in physical activity when parents and peers also participate; physical education teachers do not influence the adolescents' level of physical activity.