Do peers in childhood influence the development of noncognitive skills? Despite a large literature on peer effects, this question remains largely unanswered. In this paper, we use Big Five Inventory measures of personality traits linked to the administrative records of children randomly assigned to primary school classrooms. We find that children exposed to “left-behind” classmates whose parents have temporarily migrated away for work become less conscientious, agreeable, and emotionally stable. These impacts mirror the traits of their left-behind peers with whom children become friends.