Principals' influence on teachers is an important route through which principals may affect student outcomes. The advent of teacher evaluation systems allows estimations of principal contributions to teacher effectiveness. Employing data from the District of Columbia Public Schools, this paper examines how principals vary in their contributions to improvements in teacher effectiveness, and the relationship between this measure of principal quality and the measures DCPS employs to assess principals' contribution to teacher development. Results indicate that principals are associated with meaningful changes in teacher development. These estimated contributions are modestly correlated with DCPS's measures of principal quality.