While the importance of effective principals is undisputed, few studies have identified specific skills that principals need to promote school success. This study draws on unique data combining survey responses from principals, assistant principals, teachers, and parents with rich administrative data to determine which principal skills correlate most highly with school outcomes. Factor analysis of a 42-item task inventory distinguishes five skill categories, yet only one of them, the principals' Organization Management skills, consistently predicts student achievement growth and other success measures. Analysis of evaluations of principals by assistant principals supports this central result. The analysis argues for a broad view of principal leadership that includes organizational management skills as a key complement to the work of supporting curriculum and instruction. (Contains 18 notes, 3 figures and 6 tables.)