One of the simple truths about mentoring young people is that it is a complex activity, one that brings a wide variety of individuals together, often during moments of hardship or transition, to create something new, unique, and personally meaningful. Like all human relationships, mentoring relationships require not just initial connection, but also care and maintenance over time and a set of conditions that allow them to thrive and work toward a common purpose. Although the concept of mentoring is as old as civilization itself (and certainly embedded into the culture and ways of knowing of many communities), one of the defining features of the modern youth mentoring movement is the use of scientific research to better understand and scale these complex relationships -- what makes them work, how they support individuals' development, and how even more of these relationships than the world inhabits might produce naturally. This Research Agenda highlights four key areas for future mentoring research that MENTOR believes will help strengthen the mentoring movement over the next decade: (1) Increase research on the role of mentoring in supporting youth identity development and combating loneliness and isolation; (2) Study innovative strategies to expand natural mentoring relationships within systems and institutions; (3) Increase actionable research on mentoring that occurs in diverse programmatic settings; and (4) Invest in more research on the societal-level changes that are produced by mentoring relationships. These calls to action were developed in collaboration with a working group of leading researchers, practitioners, and thought-leaders and this document reflects their consensus on what is most critical to study in the years ahead so that the power of mentoring relationships can benefit even more young people. [For the summary, see ED614831.]