In this article, we explore the assisted performance that mentor teachers develop to help novice teachers learn to teach through analysis of the collaboration of a first-grade novice teacher and her mentor teacher in the context of the former's learning to teach mathematics in Shanghai, China. We briefly examine the possibilities and challenges associated with mentoring, describe how the novice's teaching changed over the course of her first year of teaching in the direction mathematics education reformers advocate, and explore the ways in which the mentor contributed to that development. The mentor contributed to the novice teacher's progress for several reasons. First, she developed and implemented a clear and consistent focus throughout different stages of the novice's learning and employed forms of mentoring that aligned with the kind of teaching reformers advocate. Second, the mentor also modeled, analyzed, and reflected on such mathematics teaching. Third, she defined and refined the zones of the novice's proximal development in learning to teach and pushed her to move gradually from one zone toward the other until she was able to function independently.