This paper presents a cost-benefit analysis of the Communities That Care (CTC) prevention system, a public health approach to reducing risk, enhancing protection, and reducing the prevalence of adolescent health and behavior problems community wide. The analysis is based on outcomes from a panel of students followed from Grade 5 through Grade 8 in a randomized controlled trial involving 24 communities in 7 states. Previous analyses have shown that CTC prevented the initiation of cigarette smoking, alcohol use, and delinquency by the end of 8th grade in CTC communities compared to controls. This paper estimates long-term monetary benefits associated with significant intervention effects on cigarette smoking and delinquency as compared to the cost of conducting the intervention. Under conservative cost assumptions, the net present benefit is $5,250 per youth, including $812 from the prevention of cigarette smoking and $4,438 from the prevention of delinquency. The benefit-cost ratio indicates a return of $5.30 per $1.00 invested. Under less conservative but still viable cost assumptions, the benefit-cost ratio due to prevention of cigarette smoking and delinquency increases to $10.23 per $1.00 invested. Benefits from CTC's reduction in alcohol initiation as well as broader inclusion of quality-of-life gains would further increase CTC's benefit-cost ratio. Results provide evidence that CTC is a cost-beneficial preventive intervention and a good investment of public dollars, even under very conservative cost and benefit assumptions.