This study provides new evidence on the importance of school leadership by estimating individual principals’ contributions to growth in student achievement. Our approach is quite similar to studies that measure teachers’ “value added” to student achievement, except that the calculation is applied to the entire school. Specifically, we measure how average gains in achievement, adjusted for individual student and school characteristics, differ across principals—both in different schools and in the same school at different points in time. From this, we are able to determine how much effectiveness varies from one principal to the next.