A growing body of research points to the substantial influence of principals on school conditions and students' learning. But how can principals learn to be good leaders? And are there professional learning strategies that make a difference in principal effectiveness? This study, which is part of a larger project synthesizing the research on principal learning, provides some new insight into these questions. Most studies of principal professional learning have examined the efficacy of particular programs rather than the mix of program features and experiences to which principals have had access. To begin to address this gap in the literature, the authors conducted a study of the relationship between principals' learning opportunities (both preservice preparation and in-service professional development) and key outcomes for the teachers and students in their schools. The authors examined both teacher retention and student achievement gains in English language arts and mathematics. This study offers a new perspective on the efficacy of professional learning by using detailed data from a large, representative sample of principals directly linked to individual-level information from the teachers and students in their schools. [This report was written with Anne Podolsky and Stephanie Levin. For the research brief, see ED626544.]