Most children and adolescents who engage in antisocial behaviour and delinquency do not continue to display these behaviours in adulthood. Therefore, there are factors in the natural environment that protect children-at-risk from becoming adult offenders. However, few studies have examined protective factors and the conceptual and methodological foundations for studying protective factors remain poorly developed. This article presents a review of empirical studies of factors that prevent antisocial and criminal behavior and the conceptual, limits and methodological issues that determine what factors are identified. Results show that a large number of protective factors have been identified. The lack of consistency in the findings reflects the exploratory nature of the studies and the virtual absence of replication studies. Among the methodological issues addressed, the importance of population characteristics is highlighted. Based on findings from the review, implications for future studies of protective factors are proposed, including the importance of taking account of the heterogeneity of children and adolescents at risk to offend in adulthood. Knowledge of protective factors is needed to inform policy decisions regarding investment in early childhood programs aimed at preventing criminal behaviour.