Teacher Leadership has been advanced as a solution to numerous educational issues faced such as ineffective professional learning and school improvement. However, not much is known about how teacher leadership can be nurtured through in-service professional learning programs. Utilizing survey, document analysis, and semi-structured individual interviews, this sequential mixed methods study explored (1) the process of professional learning for teacher leadership of participants in two programs, (2) the abilities that participants believe the programs helped them gain and the leadership practices they engaged in, (3) and the factors that enabled or impeded them from developing leadership capacity and practicing leadership within their school context. The findings showed that engaging in collaborative job-embedded learning within a multidisciplinary cohort, while receiving supportive and need-based coaching, fosters teacher leadership learning, and engagement in action research that develops teachers' agency for change and empowers them as professionals, is conducive to the development of teachers' leadership capacity. The results also indicated that certain leadership abilities, such as practicing and facilitating collaboration and providing professional learning to colleagues, can be acquired through programs that aren't specifically designed to build teacher leadership capacity. However, other abilities related to policy advocacy, engaging in data-driven decision-making, building partnerships, and engaging in system-thinking and whole-school decision-making require targeted coaching and focus to develop. Finally, the school context, national cultural context, and policy context were found to play a key role in learning about and practicing leadership. This study contributes added knowledge to professional learning program designers about elements that are conducive to the development of teacher leadership capacity and to administrators and policymakers about essential conditions that need to be present for teacher leadership to thrive in schools. [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://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]