In this report, we review current scientific knowledge on the topic of youth mentoring, including what is known about relationships and programs, and their interface with organizations and institutions. Two primary conclusions can be drawn from this review. First, mentoring relationships are most likely to promote positive outcomes and avoid harm when they are close, consistent, and enduring. Second, to date, programs have achieved only limited success in their efforts to establish and sustain such relationships. This is evident in a modest and inconsistent pattern of effects on youth outcomes, well-documented implementation problems, and a lack of compelling evidence of cost-effectiveness. We also review public policy issues in the field, focusing on factors underlying the popularity of youth mentoring in the US and recent efforts to extend its reach. We argue that these factors have had undesirable consequences that include decreasing intensity and infrastructure support for youth mentoring programs as well as a failure to take advantage of the full range of opportunities to cultivate and sustain mentoring relationships across different contexts of youth development. We call for a better alignment of research and practice in the area of youth mentoring, recommending policies that (a) promote evidence-based innovation, rigorous evaluation, and careful replication in dissemination for youth mentoring programs, and (b) encourage intentional and scientifically informed approaches to mentoring across the full-spectrum of youth-serving settings. (Contains 1 table and 1 figure.)