While teacher coaching is an attractive alternative to one-size-fits-all professional development, the need for a large number of highly skilled coaches raises potential challenges for scalability and sustainability. Collaborating with a national teacher training organization, our study uses administrative records to estimate the degree of heterogeneity in coach effectiveness at improving teachers' instructional practice, and specific characteristics of coaches that explain these differences. We find substantial variability in effectiveness across individual coaches. The magnitude of the coach-level variation (0.2 to 0.35 standard deviations) is close to the full effect of coaching programs, as identified in other research. We also find that coach-teacher race/ethnicity-matching predicts changes in teacher practice, suggesting that the relational component of coaching is key to success.