School-based mentoring is one of the fastest growing forms of mentoring in the US today; yet, few studies have rigorously examined its impacts. This landmark random assignment impact study of Big Brothers Big Sisters School-Based Mentoring is the first national study of this program model. It involves 10 agencies, 71 schools and 1,139 9- to 16-year-old youth randomly assigned to either a treatment group of program participants or a control group of their non-mentored peers. Surveys were administered to all participating youth, their teachers and mentors in the fall of 2004, spring of 2005 and late fall of 2005. The report describes the programs and their participants and answers several key questions, including: Does school-based mentoring work? What kinds of mentoring experiences help to ensure benefits? How much do these programs cost? Our findings highlight both the strengths of this program model and its current limitations and suggest several recommendations for refining this promising model--recommendations that Big Brothers Big Sisters agencies across the country are already working to implement. Eight appendixes are included: (1) Methodology; (2) Impact Analyses; (3) Participating Schools; (4) Associations between Match Support and Training and Indicators of Match Success; (5) Effect Sizes; (6) Does BBBS SMB Provide Different Benefits for Different Groups of Littles?; (7) Follow-Up Analyses to Further Explore Associations between Match Length, Relationship Quality and Outcomes; and (8) Outcome Trajectories for One-School-Year and 15-Month Littles. (Contains 117 endnotes, 5 figures, and 42 tables.) [This report was written with Linda Z. Jucovy.]