This meta-analysis investigated the long term effects of prevention programs conducted during early and middle childhood on criminal offending during adulthood. The analyses included 3611 participants in 9 programs. The effect size for adult criminal offending was significant, but small in magnitude (OR=1.26; 95% CI=1.06-1.50, p=.011). The effects of the programs on positive outcomes (academic attainment and involvement in productive activity, such as being engaged in school or work) were somewhat larger and more consistent than effects on crime (OR=1.36, 95% CI=1.20-1.55, p<.001). Several participant and program characteristics moderated the effectiveness of (early) prevention. Children who were more at-risk and those from a lower SES benefited more. Shorter, but more intensive programs, and programs that focus on social and behavioral skills, rather than on academic skills or family support, tend to produce larger effects. Taken together, these results indicate that early prevention programs can help put children on a more positive developmental trajectory that is maintained into adulthood, but there is still no convincing evidence that they can prevent adult crime. Implications of the findings for research, policy and clinical practice are discussed.