This volume presents research from the PROSPER Peers project, which for over a decade has sought to illuminate how adolescent friendship networks channel and facilitate the spread of developmental outcomes such as substance use, other risky behaviors, mental health problems, and educational success. In addition, we have probed the role of friendship networks in extending the impact of school and family-based prevention programs aimed at reducing substance misuse and improving adolescents' futures. The chapters presented here bring together results from PROSPER Peers' 50+ publications along with new analyses and findings. All of this work was made possible by tracking the friendship networks and behaviors of thousands of students in twenty-seven Iowa and Pennsylvania communities across middle and high school--