This study addresses the question of how undergraduates with an opportunity to serve as teachers, or "peer facilitators", at the college level think about and approach teaching. Peer facilitators in the "Gateway Science Workshop" Program at Northwestern University serve in a teaching role for one to two years, leading weekly, small group workshop sessions for students in their first year "gateway" science, technology, engineering, and mathematics (STEM) courses. The research took place within a large, funded intervention aimed at reducing the gap in performance and retention between undergraduate minority and majority science students. The study found that the sample of 19 peer mentors conceived of and approached their teaching task in distinctly different ways, adopting a teaching-centered or a learning-centered framework that changed over time with gains in experience. The developments documented over the course of their teaching experience have important implications for understanding how undergraduates think about learning and how they understand teaching. (Contains 2 figures.)