Instructional coaching is an attractive alternative to one-size-fits-all teacher training and development in part because it is purposefully differentiated: Programming is aligned to individual teachers’ needs and implemented by an individual coach. But, how much of the benefit of coaching as an instructional improvement model depends on the specific coach with whom a teacher works? Collaborating with a national teacher training and development organization, TNTP, we find substantial variability in effectiveness across coaches in terms of changes in pre-service teachers’ instructional practice (roughly 0.25 to 0.3 standard deviation from our preferred sample and models). The magnitude of coach effectiveness heterogeneity is quite similar to average coaching program effects on teaching practice identified in other research. Through a set of alternative model specifications and permutation tests, we rule out the possibility that our estimates of coach effectiveness heterogeneity are driven by nonrandom sorting of coaches to teachers, at least on observable characteristics available in our data, as well as the possibility that these estimates are simply statistical noise. These findings suggest that identifying, recruiting, and supporting highly skilled coaches will be key to scaling instructional coaching programs.