This article presents findings from a two-year, qualitative study of K-12 teacher Fellows involved in a grant-funded professional development program. Aiming to foster sustainable change in districts and support emergent teacher leadership, the program enabled Fellows to collaboratively reflect on practice and develop as teacher leaders who lead informally from the classroom. Using activity theory as an analytical lens, the following main themes emerged in the data: (1) Teacher leaders have complex definitions of teacher leadership that parallel teaching beliefs; (2) Teacher leaders need strong communication skills to collaborate within different contexts; (3) Teacher leaders benefit from work on vertical articulation; and (4) School culture and administrative support influences teacher leadership. We explore the implications for professional development programs in districts, and in particular, those that address the need to cultivate teacher leadership.