This study follows leadership candidates through the first phase of a comprehensive effort to reform master's-level principal preparation at a large, urban California university. The reforms placed an 18-month field experience at the center of candidates' preparation. Researchers sought to capture the changes over time in candidates' beliefs about school leadership, commitment to the work of school leadership, knowledge of leadership practices that support improved teaching and learning, and capacity to apply those practices. The results reveal marked changes in the majority of candidates' understandings of school leadership. They came to see the work as complex, with all aspects interrelated. They developed deeper recognition of the leader's role in fostering trust and relationships, encouraging collaboration, and building leadership capacity within schools. They conceptualized data as powerful evidence to stimulate urgency for change, and they articulated greater confidence as leaders and change agents. Finally, candidates demonstrated increased understanding of, and ability to enact, specific leadership practices aimed at improving learning results for students in their schools. (Contains 1 table.)