This paper uses random assignment of students to investigate the impact of study groups and roommates on academic achievement. We find that informal social interaction with roommates has a significant positive impact on academic achievement, while study group peers have no discernible impact, a result driven by group heterogeneity in ability. We also find that lower-ability students benefit from high-ability students but not vice versa. © 2015 by the President and Fellows of Harvard College and the Massachusetts Institute of Technology.