The Instructional Leadership Corps (ILC) is a California collaborative teacher professional learning project in which expert teachers organize local professional development to spark iterative changes in practice. Launched in 2014, ILC is a joint effort of the California Teachers Association (CTA), the National Board Resource Center (NBRC), and the Stanford Center for Opportunity Policy in Education (SCOPE). The ILC changes the paradigm for teacher learning from one dependent on outside consultants, who often conduct one-shot workshops before they leave for the next district, to one that engages local professionals who have been trained and supported to lead ongoing learning within their own districts--and, in many cases, to carry that learning to other schools and districts in their region. In this study, the authors sought to discover how ILC teams in different settings gained traction and began to transform professional learning opportunities in their communities and regions, often addressing long-standing problems of practice and inequities in children's access to high-quality instruction. Given that practitioner-led professional learning has often failed to gain a toehold in districts where teacher leaders are appointed but not integrated into the work of the schools, we wanted to understand what has enabled the work of the ILC to grow and become rooted in various communities. They examined the strategies used by ILC leaders both in conducting professional development and in connecting their work to the broader efforts of their districts and counties. They also examined the perceived impacts on practice for teacher participants. The authors interviewed ILC leaders; participating teachers; and school, district, and county administrators. They observed professional learning workshops, statewide conferences, and conferences organized by ILC teams. They also observed classrooms of teachers who participated in workshops led by ILC leaders. [For the research brief, see ED603446.]