This paper studies how the effectiveness of teachers varies by classroom composition. We combine random assignment of teachers to classrooms with rich measures of teacher effectiveness based on a popular observational protocol, Framework for Teaching, to overcome key endogeneity concerns related to measurement and matching. We find that complementarities between classroom composition and teaching practice play a significant role in student achievement. We identify two main mechanisms that are driving this result: (1) negative interactions between challenging and/or student-centered practices and heterogeneity in classroom prior achievement, and (2) positive interactions between classroom management skills and average classroom prior achievement. Our findings illustrate the multidimensional nature of teacher effectiveness and have important implications for prescribing teaching practice and evaluating teachers. Simulations show that teacher rankings change substantially simply from within-school classroom reallocations, suggesting the need for caution when using popular teaching evaluation rubrics in high-stakes settings.