The University of Rochester, in partnership with the Rochester City School District and the Rochester Museum and Science Center, are developing one cohort of 24 K-12 master teachers in science and/or mathematics in this high-need school district, with the long-term goal of building capacity for sustaining high quality mathematics and science teaching. Master Teaching Fellows are engaging in coursework and mentored practice focused on increasing the teachers' knowledge and skills in STEM content, pedagogy, reform-based instructional materials and assessments, professional development, and leadership. The Fellows serve as role models, professional development providers, and instructional leaders for current and prospective teachers in the district. The five-year professional development program for the Fellows focuses each year on a different dimension of their roles as master teachers, with specific emphases on examining their own teaching practices, serving as mentors to other in-service and pre-service STEM teachers, providing professional development through courses and summer institutes, serving as change agents promoting STEM education innovations in the school district, and sustaining STEM education reform. The professional development program is being carried out through graduate-level courses, mentored field experiences pertinent to each year's focus, and monthly leadership seminars. A subset of Master Teaching Fellows is also completing additional requirements necessary to obtain state certification as a school or district leader and/or completing an Ed.D. program to further prepare them to serve specific roles in the district's STEM reform plan. The final two years of the project are designed to provide a transition to post-funding so as to build long-term sustainability, by engaging Fellows in work alongside district leaders in developing and implementing STEM education reform initiatives. The program evaluation is assessing the impact of the program on the Fellows' effectiveness as STEM teachers and leaders in order to advance knowledge about what it takes to prepare master teachers who can support other in-service and pre-service teachers in implementing high-quality STEM instruction in urban schools.