Mathematics teacher education as a field of research explores a wide range of issues and questions relating to the preparation and development of mathematics teachers. Much of the research in mathematics teacher education is conducted by the individuals who are responsible for this preparation and development, that is, university-based mathematics teacher educators whose role it is to educate prospective and practicing teachers of mathematics, help initiate their growth and development throughout their professional lives, and to advance the understanding of the field. In research reports from researchers who are also mathematics teacher educators, we tend to read about “what they observed of the teachers they studied and their analysis of the resulting development of teaching practice” (Jaworski, 2021, p. 422) and expertise. Less commonly do we read about what they observed of themselves and other mathematics teacher educators and their analysis of the resulting development of their own teaching practices and expertise (Jaworski, 2021). It is the development of expertise in mathematics teacher educators that is the focus of this book with all of the chapters authored by practicing university-based mathematics teacher educators.