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 teachers’ classroom practice (0.43 standard deviations). The magnitude of coach effectiveness heterogeneity is close to average coaching program effects identified in other research. These findings suggest that identifying, recruiting, and supporting highly skilled coaches will be key to scaling instructional coaching programs.