Teacher coaching has emerged as a promising alternative to traditional models of professional development. The authors review the empirical literature on teacher coaching and conduct meta-analyses to estimate the mean effect of coaching on teachers’ instructional practice and students’ academic achievement. Combining results across 44 studies that employ causal research designs, we find pooled effect sizes of .58 standard deviations (SD) on instruction and .15 SD on achievement. Much of this evidence comes from literacy coaching programs for pre-kindergarten and elementary school teachers. Although these findings affirm the potential of coaching as a development tool, further analyses illustrate the challenges of taking coaching programs to scale while maintaining effectiveness. Coaching effects in large-scale effectiveness trials with 100 teachers or more are only half as large as effects in small-scale efficacy trials. The authors conclude by discussing ways to address scale-up implementation challenges and providing guidance for future causal studies.