Mentoring is one of the most popular social interventions in American society, with an estimated three million youth in formal one-to-one relationships. Studies have revealed signiﬁcant associations between youth involvement in mentoring relationships and positive developmental outcomes. These associations are modest, however, and depend on several intervening processes. Centrally important is the formation of close, enduring connections between mentors and youth that foster positive developmental change. Effects of mentoring programs likewise typically have been small in magnitude, but they increase systematically with the use of program practices likely to support relationship development. Gaps between research and practice are evident both in the indiscriminate use of the term mentoring in the prevention ﬁeld and in a focus on the growth and efﬁciency of mentoring programs at the expense of quality. Continued expansion of effective mentoring will require a better alignment of research and practice.