Using longitudinal elementary school teacher and student data, we document that students have larger test score gains when their teachers experience improvements in the observable characteristics of their colleagues. Using within-school and within-teacher variation, we further show that a teacher's students have larger achievement gains in math and reading when she has more effective colleagues (based on estimated value-added from an out-of-sample pre-period). Spillovers are strongest for less-experienced teachers and persist over time, and historical peer quality explains away about twenty percent of the own-teacher effect, results that suggest peer learning.