The review focuses on the following mentoring-related topics: 1) the overall contributions of mentoring to reducing or preventing delinquency behavior; 2) factors that may condition or shape the extent to which mentoring has effects on delinquent behavior; 3) processes that may be involved in determining the effects of mentoring on delinquent behavior; and 4) the extent to which mentoring strategies focused on preventing or reducing delinquent behavior have reached at-risk youth, been implemented effectively, and been adopted and sustained by settings. The research reviewed suggests, but does not definitively identify, several factors that may influence the effectiveness of mentoring in preventing or reducing delinquent behavior. One possibility is that prior involvement in the courts may influence youth to resist rather than to receive mentors, who may be viewed as an extension of a restrictive, oppressive justice system; however, both “mattering” (defined as being noticed, needed, and having someone concerned about your welfare) and having one’s positive traits be acknowledged and helpful to others are core indicators of positive development, including setting and pursuing positive goals. When assessing the outreach, quality, and sustainability of mentoring programs directed toward preventing or reducing delinquent behavior, they have proven to be only partially successful. The degree of coordination between juvenile justice settings and mentoring programs appears to be an important component in determining the delinquency prevention/reduction effect of mentoring.