Each of the three studies used a longitudinal design in which a sample of children and/or adolescents was selected and then followed over time to chart the course of their development. The studies oversampled youth at high risk for serious delinquency; however, because the studies used statistical weighting, the samples were representative of the broader population of urban adolescents. The studies produced data on delinquent behavior of sample subjects from 1987 to the present (2003) and have included more than 4,000 subjects who have ranged in age from 7 to 30. The samples included a strong representation of serious, violent, and chronic offenders. To date, more than 100,000 personal interviews have been conducted with participants, and additional information and data have been obtained from schools, police, courts, social services, and other agencies. In summarizing findings on a few of the many topics addressed in relation to the causes and correlates of delinquency, some of the data are specific to only one of the projects while others pertain to two or all three of the projects. Three key topics addressed in this article are childhood aggression, developmental pathways to delinquency, and the overlap of problem behaviors. The studies have investigated a host of risk factors the involve child behavior, family functioning, peer behavior, school performance, and neighborhood characteristics that precede and potentially lead to delinquency. Findings on just two topics, child maltreatment and gangs, are summarized in this article. The final two sections of the article focus on responding to delinquency and the implications of the findings for delinquency prevention. The consistent finding from these studies was that in general, arrest had little impact on subsequent delinquent behavior, other than to increase it. Regarding the use of services to help deal with delinquent behavior, less than half of the parents of seriously delinquent boys received any help from community service agencies, and only one-quarter of the parents of delinquent boys received help from a mental health professional. Regarding implications of the findings for prevention, overall it appears that the judicious use of early interventions known to have long-term effectiveness is warranted. Also, a full range of developmentally appropriate and scientifically validated programs is needed to reduce the risk of recidivism for serious offending. Maltreatment that occurs during adolescence is apparently a substantial risk factor for later delinquency. This suggests the need for enhanced services for adolescent victims. Further, a general strategy for reducing youth crime should be mindful of the significant impact of gang membership on serious and violent delinquency. Intervening with gang members must be an important component in a strategy to reduce a community's level of youth crime and violence. 5 notes and 53 references