This study extends previous understanding of the association between adolescent extracurricular activity participation and adjustment by asking whether participation in school-based extracurricular activities is associated with lower substance use and depression, higher grades and academic aspirations, and more positive attitude toward school. In addition, it examines the role of life events stress and time spent in activities as potential moderators of the association. Data come from an ethnically diverse sample of adolescents from 6 California high schools (N = 3,761). Cross-sectional results suggest participation is associated with more positive adolescent outcomes for all variables except alcohol use and depression, controlling for gender, ethnicity, and grade. Longitudinal analyses are consistent with these findings. Cumulative participation over a 3-year period predicts adolescent outcomes controlling for both Time 1 outcomes and demographic characteristics. In addition, within-person analyses reveal year-to-year covariation of participation and positive outcomes.