This thesis, comprised of two studies, used a 1-year longitudinal design to explore which social ecological factors prospectively predict moderate-to-vigorous physical activity (MVPA) among adolescents reporting a decline in MVPA (n = 195 and n = 131). Regression analyses identified which variables predict (a) MVPA one year later, and (b) a decline in MVPA over one year. MVPA was measured in days/week (study 1) and metabolic equivalent (MET) hours/week (study 2). In both studies, initial MVPA was the strongest predictor of MVPA one year later. In study 1, perceived availability/accessibility of facilities/supplies was the only other significant predictor of MVPA one year later. In study 2, perceived availability/accessibility of facilities/supplies and self-efficacy to overcome social environment barriers were additional predictors of MVPA one year later. Only social support from friends predicted a decline in MVPA (MET hours/week), such that those reporting more social support were more likely to decrease MVPA.