This article examines how the MAT [Master of Arts in Teaching] program in Earth Science at the American Museum of Natural History was conceptualized and enacted within its institutional context. We argue that the program was completely consistent with the museum's public and democratic institutional logic, as reflected in funding, staffing, location, and regulation. Further, we suggest that learning to teach was conceptualized as a process combining informal science learning with professional learning at the nexus of multiple communities of practice. Together, these arguments show that science teacher preparation at the AMNH was a natural outgrowth of the museum's commitment to educating the public in a democratic society.