This study examines the role of the high school principal in building teacher leadership capacity as a potential tool for use in meeting school improvement goals. High schools were chosen as the focus of this study for two reasons. First, high school students generally do not perform as well as their elementary and middle school peers on standardized tests. Additionally, the pressure to produce 21st century learners is most intense at the high school level. If high school principals are to achieve reform expectations, they will be most successful if they can tap into the expertise of the teachers already working on their campuses. Existing literature on teacher leadership and principal leadership as it relates to developing and supporting teacher leadership, building leadership capacity, and distributing leadership was reviewed. Interviews and focus groups conducted with 15 teacher leaders and three principals in one high performing Southern California school district provided the data that informed the study. Both teacher and administrator perspectives were captured and analyzed. The analysis revealed ten themes : empowerment, culture of continuous improvement, collaboration, relationships, clear expectations, professional development, support for teachers, vision, organizational structures, and challenges. The study reinforced the notion that principals set the tone for creating a culture of teacher leadership and have several tools for building leadership capacity at their disposal