Communities across the country have a vested interest in making sure that young people develop into healthy productive citizens and avoid behaviors that can jeopardize their own health and well-being and threaten the well-being of their families and neighborhoods as well. Substance abuse and delinquency are prime examples of behaviors that get in the way of positive development. Researchers in the field of prevention science have identified a number of factors that make it more likely or less likely that a young person will adopt problem behaviors. Prevention scientists have drawn on these findings to design programs aimed at preventing youth from getting caught up in delinquency, drug use, and other problem behaviors, and they have evaluated these programs using rigorous scientific criteria. In spite of these advances, tested and effective approaches to help youth develop into productive citizens and avoid problem behaviors have not been used widely in schools and communities, and efforts to establish effective prevention systems have been limited. The Communities That Care (CTC) system was developed to address this gap. This "Research Brief" describes the Communities That Care prevention system, the steps involved in implementing this system, and major findings from a community randomized controlled trial (considered the "gold standard of research") of Communities That Care. That study followed a panel of students from fifth through tenth grade. By the end of eighth and tenth grades, those in Communities That Care sites were less likely to start smoking cigarettes, to start drinking, and to start engaging in delinquent behavior than were their counterparts in control communities that did not use the CTC system.