The purpose of this study was to understand what knowledge, skills, and practices teacher leaders learn from exercising leadership with other teachers in their school settings and how, if at all, they learn from experience and in communities of practice (CoPs), that is, groups of people with shared interest in a particular domain (Lave & Wenger, 1991). The sample was comprised of two public high school principals, 15 teacher leaders nominated by one principal, and six by the other. The researcher conducted approximately two hours of interviews with principals and 20 hours with teachers. Within and cross case analysis facilitated the comparison of commonalities and differences between individuals. Teacher leaders reported on learning four clusters of knowledge (i.e., teaching and learning, work context, adult learning principles, research) and two clusters of skills (i.e., collegial interaction, big picture) to perform seven areas of practice (i.e., professional development, participation in school change or improvement, curriculum work, coordination or management, pre-service teacher education, contribution to profession, parent or community involvement). Teacher leaders learned the necessary knowledge, skills, and practices as they stepped up in CoPs, working alongside each other and moving between identities. Yet, while CoPs availed conditions that facilitated their learning, teachers also needed to "create" those very conditions, an act of leadership in itself. A majority also learned from experience--reflecting-in-action and reflecting-on-experience individually and collectively. In addition, differences in what and how teachers learned emerged between the two schools and across participant demographics. Teachers indicated eleven conditions in the school culture, five in the school structure, and five supports from principals as facilitators of their learning and practice. They cited seven conditions in the school context, seven personal factors, and two district constraints as hindrances, which also became the impetus for learning, as challenging situations often pushed teachers to step up. Recommendations included for current and prospective teacher leaders to develop a repertoire of leadership knowledge and skills, for administrators to build collaboration time into the schedule, for educators and development practitioners to support teachers in reflective practices, and for researchers to conduct a similar study in different school settings. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]