School-based mentoring (SBM) has exploded in popularity in recent years: Today approximately one fourth of the youth mentoring programs in the country use a school-based format (Herrera, Grossman, Kauh, Feldman, & McMaken, 2007). In SBM, a K-12 student is paired with an adult from the community or an older (usually high school) student in a supportive one-to-one relationship at the school site. The enthusiastic growth of this model has been fueled, in part, by some of the widely reported successes of community-based mentoring in the mid-1990s (Tierney & Grossman, 2000), which indicated adult mentors could have a positive impact on many aspects of a youth's social and academic life. Naturally, both youth-serving nonprofits and school districts wondered if similar impacts could be achieved by delivering mentoring at the school site, capitalizing on existing school infrastructure and staffing to help manage the program and support the mentoring relationships. While some studies have questioned the efficacy of SBM (Bernstein, Rappaport, Olsho, Hunt, & Levin, 2009; Herrera et al., 2007), the experience of Education Northwest's National Mentoring Center (NMC) indicates that such mentoring programs can work well. Further, they have tremendous potential to help students in a number of academic and psychosocial domains, provided programs follow the emerging guidance provided by recent research. The NMC has been at the leading edge of the expansion of SBM since 1999, serving as a training and technical assistance provider for national mentoring initiatives funded by the U.S. Department of Education, the Office of Juvenile Justice and Delinquency Prevention, and the Corporation for National and Community Service. This "Lessons Learned" taps into NMC's experience to focus on what people consider the critical ingredients of successful SBM programs, as well as common pitfalls to avoid. These "lessons" will be most valuable to schools or districts that are contemplating starting a SBM program, although sites with existing programs may find this information helpful in the restructuring or refining of their mentoring services.