This chapter synthesises the contributions of part 1 and part 2 of this book, highlighting the meaning of Mathematics Teacher Educator (MTE) expertise and its development, as well as offering new insights and directions for future research in this field. The research presented in these chapters underscores the multidimensional nature of MTE expertise and its dynamic evolution within the context of MTEs’ professional work. Methodologically, the studies adopt self-study and narrative approaches, often in combination with other theoretical constructs, to examine the process of MTE expertise development. However, several issues remain open for further investigation—particularly the role of theory and research in shaping MTE expertise, and the importance of integrating social, cultural, and ecological dimensions into our understanding of this expertise.