Findings from an extensive four-year program with a single cohort of elementary and early middle school mathematics instructors highlight several important themes that can inform other efforts in developing teacher leaders. The authors describe the Arizona Master Teachers of Mathematics (AZ-MTM) program as an example that supports the development of mathematics teacher leaders over a sustained period of time. AZ-MTM has two complementary and intertwined strands: the professional development apprenticeship (PDA) and the teacher study group. Felton and Page offer highlights of AZ-MTM such as: (1) remaining closely connected to issues of practice in the classroom and in leadership activities; (2) supporting the development of a professional learning community committed to excellent mathematics teaching; and (3) striking a balance between providing common experiences to develop a professional learning community, and allowing room for individualization within the program.