High schools have recently become a popular source of mentors for school-based mentoring (SBM) programs. The high school Bigs program of Big Brothers Big Sisters of America, for example, currently involves close to 50,000 high-school-aged mentors across the country. While the use of these young mentors has several potential advantages, their age raises questions about their capacity to be consistent, positive role models, and, in turn, their potential to yield strong impacts for the youth they mentor. With support from The Atlantic Philanthropies and in collaboration with Big Brothers Big Sisters of America, P/PV set out to address these questions using data from our large-scale random assignment impact study of Big Brothers Big Sisters SBM (Herrera, et al. 2007). We found that, on average, high school students were much less effective than adults at yielding impacts for the youth they mentor. However, our research identified several program practices that were linked with longer, stronger and more effective high school mentor relationships. High School Students as Mentors stresses the need for programs with high school volunteers to use the inherent strengths of these volunteers and, at the same time, meet their distinct needs. Big Brothers Big Sisters of America is already initiating most of the changes suggested in the study in its high school Bigs program; it has convened a group of six of its strongest Big Brothers Big Sisters agencies to review these and other findings and share their own experiences and strategies in an effort to strengthen their model.