One of the early intervention programs assessed consists of home visits by child-care professionals beginning before birth and extending through the first 2 years of childhood, followed by 4 years of day care. The second program consists of training for parents and therapy for families with very young school-age children who have shown aggressive behavior or otherwise begun to "act out" in school. Another intervention program involves using 4 years of cash and other incentives to induce disadvantaged high school students to graduate. The fourth program consists of monitoring and supervising high-school-age youth who have already exhibited delinquent behavior. The effectiveness of the programs was assessed by examining reductions in arrest or rearrest rates. The costs assessed were based only on the costs of program operations and not on the savings realized by not having to eventually imprison those youths diverted from criminal careers. Of the four programs, graduation incentives for high- risk youth appears to hold the most promise. The preliminary analysis suggests that the cost of preventing serious crimes with this program is approximately $4,000 per crime. The parent- training intervention could be relatively cost-effective over the long term at a cost of approximately $6,500 per serious felony prevented. The early home-visit and day-care intervention works with high-risk youth and their families during the first 5 years of childhood. This intervention requires large expenditure to affect large numbers of youths Based on current best estimates of program costs and benefits, investments in some interventions for high-risk youth may be several times more cost-effective in reducing serious crime than long mandatory sentences for repeat offenders. 8 figures and 23 tables