Preparation and professional development can help principals be more effective leaders, but little research has examined what specific approaches are most useful. A study of 462 California principals and the teachers and students in their schools shows that high-quality preservice preparation for principals is associated with stronger teacher retention, and that overall access to professional development--as well as specific learning about instructional leadership--is strongly associated with student achievement gains in both mathematics and English language arts. Underrepresented students of color benefit the most from their principals' opportunities to learn, as do the students of early-career professionals. [This report was written with Anne Podolsky and Stephanie Levin. For the full report, see ED626543.]