Objective : To evaluate social support and theory of planned behavior (TPB) constructs in explaining physical activity in adolescent girls. Methods : One thousand seven hundred ninety-seven 8 th -grade girls completed a survey measuring social provisions, family support, TPB constructs, moderate-to-vigorous physical activity (MVPA), and team sport involvement (TSI). Results : Structural equation modeling (SEM) indicated statistically significant direct effects from TPB constructs and social provisions to intention; social provisions, family support, and intention to MVPA; and social provisions and family support to TSI. Conclusions : Social variables play a greater role in explaining TSI than in explaining MVPA in this sample.