This article summarizes published research regarding the effectiveness of mentor programs in general, and for youth in foster care specifically, as a basis for evidence-based practice in child welfare. It examines the pros and cons of mentor programs and characteristics of programs that are more or less effective for achieving specific social goals. The author explores the opportunity cost of investments in transitional mentor programs versus efforts to find permanent parents for youth aging out of care, and concludes with practice and policy implications of these findings.