Background: Adolescent moderate to vigorous physical activity (MVPA) is influenced by many factors. MVPA-promotion interventions would fare better if these multiple determinants were better understood. Purpose: To simultaneously assess overall and relative contributions of factors from personal, family, friend, school, and neighborhood contexts to adolescent MVPA. It was hypothesized that (1) key correlates would emerge in each context and (2) factors from more- versus less-proximal contexts would relate more strongly to MVPA. Methods: Students in grades 6-12 (n=2,793; mean age=14.4 [SD=2.0] years; 53% girls) were recruited from 20 Minnesota public schools in 2009-2010 to participate in the Eating and Activity in Teens 2010 study. Regression analyses conducted in 2013 examined factors related to weekly MVPA. Data were collected from adolescent participants, their parents and friends, school teachers and administrators, and GIS sources. Results: Fifty multicontextual factors explained 25% of MVPA variance for boys and 27% for girls. Personal factors (e.g., self-efficacy) were most predictive of MVPA, followed by social factors (e.g., support for PA); environmental factors (e.g., access to PA resources) were least predictive of adolescent PA. Gender differences emerged for several predictors (e.g., in mutually adjusted analyses, MVPA among girls, but not boys, related positively to distance to trails and MVPA among female friends and fathers, and related negatively to perceived barriers). Conclusions:. Stronger linkages exist between adolescent MVPA and more-proximal (personal, family, and friend) factors compared to more-distal (school and neighborhood) factors, suggesting the importance of working with adolescents, their families, and friends to promote PA. (C) 2014 American Journal of Preventive Medicine