This article describes the Boston Teacher Leadership Certificate program created to expand teachers' capacity for teacher leadership roles. The professional learning was designed to strengthen teachers' leadership skills while also paying attention to whether and under what conditions they were more successful in their roles. To monitor the program's impact, the Boston Teacher Leadership Certificate program used four data routines: comparing pre-and post-course surveys, analyzing exit slips, reviewing online discussions, and looking together at student work. These routines supported collaborative inquiry, shared ownership for the quality of the work, and learning at all levels. At the end of the professional learning, 95% of course participants indicated that the course would improve their ability to have a positive impact on others' teaching practice, and 91% felt that the course would have a positive impact on their own teaching practice.