In this paper we present a study conducted within the framework of an in-service professional development program for junior and senior high school mathematics teachers. The focus of the study is the analysis of processes encountered by the staff members, as members of a community of practice, which contributed to their growth as teacher educators. We offer a three-layer model of growth through practice as a conceptual framework to think about becoming a mathematics teacher educator, and illustrate how our suggested model can be adapted to the complexities and commonalities of the underlying processes of professional development of mathematics teacher educators.