In an era of greater school accountability, leadership matters. For decades, principals have been recognized as vital to the effectiveness of schools, but strong empirical evidence on the extent to which, and the ways in which, school leaders matter has not been available. CALDER researchers have advanced our knowledge in this area by skillfully drawing on rich state longitudinal databases. This brief synthesizes new findings on the effectiveness and distribution of principals, the characteristics of good leadership, and how best to prepare principals for this increasingly demanding job.