Benefits and costs are discussed when substance use prevention programs are proposed and evaluated, but systematic benefit-cost analysis has rarely been applied to such programs. This article argues that benefit-cost analysis can and should be used to structure evaluations of substance use prevention programs, and, in general, any program aimed at preventing dysfunctional behavior. It presents the basic ideas of benefit-cost analysis and its closely related tool, cost-effectiveness analysis. It then sketches a "blueprint" for a benefit-cost analysis of an experimental drug use prevention program intended to prevent relapse into drug use by parents receiving methadone maintenance and to lower the risk that the children of these parents will become substance users. The blueprint can help structure evaluations of other prevention programs.