It is widely believed that a good principal is the key to a successful school. Yet until very recently there was little rigorous research demonstrating the importance of principal quality for student outcomes, much less the specific practices that cause some principals to be more successful than others. 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. Our results indicate that highly effective principals raise the achievement of a typical student in their schools by between two and seven months of learning in a single school year; ineffective principals lower achievement by the same amount.