Supplemental instruction (SI)--variously known as peer-assisted learning, peer-assisted study sessions, and other names--is a type of academic support intervention popular in higher education. In SI sessions, a senior student facilitates peer learning between undergraduates studying a high-risk course. This article presents a systematic review of the literature between 2001 and 2010 regarding the effectiveness of SI. Twenty-nine studies met the inclusion criteria. Due to methodological heterogeneity and lack of consistency defining the SI treatment, qualitative synthesis methods were applied. For seven included studies, however, an effect size of SI participation on final grades was calculated, ranging from d = 0.29 to d = 0.60. The findings of the review are consistent with claims validated by the U.S. Department of Education in the 1990s that participation in SI is correlated with higher mean grades, lower failure and withdrawal rates, and higher retention and graduation rates.