Youth mentoring programs are in the limelight. Over three million young people have a Big Brother, a Big Sister, or a similar adult volunteer involved in their lives–a sixfold increase from just a decade ago–and generous federal funding continues to fuel new initiatives. This expansion speaks volumes about the faith our society places in one-on-one relationships between vulnerable young people and caring adults. But what do we know about the effectiveness of this intervention strategy? A better understanding of the research evidence for youth mentoring, including findings from reviews, evaluations, and meta-analyses, provides a basis for a more informed, practically applicable approach to strengthening youth mentoring interventions.