Drug use produces social costs in terms of increased health care, lost productivity, and other costs. The social value or social benefit of a drug-use prevention program includes the social costs that would have been incurred without the program. The study began the calculation of prevention's social benefit by estimating how much of a specific drug the average person would use over the course of a lifetime. It then estimated by what percentage a model prevention program would be expected to reduce that drug usage. Multiplying these two estimates yielded the reduction in use of the drug by the average person exposed to the prevention program. This number was multiplied by a social value that reflected the social costs averted per a given amount of drug not used. The findings for various substances were compared to determine the drugs for which the reductions were most valuable. The sum of these figures across drugs was compared with the cost of operating a prevention program. The principal finding of the study is that the benefits of model school-based drug prevention program do outweigh their costs. The best and the conservative benefit estimates account for school-based prevention's effects on four drugs: alcohol, tobacco, marijuana, and cocaine. These were the four drugs for which the evidence supported the estimates of the magnitude of lifetime reductions in use. Under the assumption that prevention programs reduce the use of other illicit drugs (heroin, methamphetmaines, and other controlled substances) by the same proportion that they reduce cocaine use, the estimated benefit per participant would increase to about $1,000. Close to 40 percent of the social value of drug-use prevention was realized through reductions in tobacco use, and over one-fourth of the value was in decreased alcohol abuse. Most of the remaining third was associated with cocaine reductions; marijuana accounted for a small fraction of the total. Implications of these findings for drug policies and dollar investments are discussed. Extensive tables and figures, appended supplementary material, and a 90-item bibliography