While the importance of effective principals is undisputed, few studies have addressed what specific skills 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 identify which principal skills matter most for 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 confirms this central result. Our analysis argues for a broad view of instructional leadership that includes general organizational management skills as a key complement to the work of supporting curriculum and instruction. Two appendices are included: (1) Factor Loadings Matrix for Principal Effectiveness Factors; and (2) Factor Loadings Matrix for Assistant Principal Effectiveness Factors. (Contains 3 figures, 5 tables and 6 footnotes.) [This paper was supported by the Stanford University K-12 Initiative.]