High quality teaching is critical to student learning, yet takes considerable time to develop in particular content areas. Students in high-poverty, urban settings are less likely to encounter experienced and trained teachers. Administrators from a large school district and university mathematics education faculty partnered and attempted to address this by designing a professional development program that prepared experienced teachers to work as specialists. Teachers engaged in intensive professional development while teaching only mathematics to students in grades 4-6. I describe the program and discuss some of the issues that arose when mathematics specialists taught only mathematics in upper elementary school classrooms. I ground the discussion in research on three years of the mathematics specialist program and share reports from teachers of their experiences.