Using a unique longitudinal dataset from Florida, we analyze the impact of classroom peers on individual student performance. Focusing on the influence of peers' fixed characteristics on individual test score gains, we control simultaneously for student and teacher fixed effects. We find some sizable, significant peer effects within nonlinear models, but not with linear specifications. We find peer effects depend on a student's own ability and on the ability of the peers under consideration. Peer effects tend to be smaller when teacher fixed effects are included, a result that suggests co-movement of peer and teacher quality within a student over time.