Boston Public Schools, winner of the 2006 Broad Prize for Urban Education, creates innovative programs to develop school leaders based on the idea that "school leadership is the single most important factor in schools' success." Teachers play an invaluable role in determining the quality of classroom instruction, but a principal is, in the words of former Boston superintendent Thomas Payzant, the "teacher of teachers" who envisions, implements, and supports high-quality learning environments. This article describes how Boston Public Schools grows district principals through a year-long fellowship program that immerses participants in the culture and challenges of the district.