The complexity of mathematics teacher educator expertise

Abstract Mathematics teacher education is now a well-established field of research, but it is only in recent years that research attention has turned towards the learning and development of mathematics teacher educators themselves. In this special issue, interest centres on mathematics teacher educator expertise and how this can be represented, developed, and studied. The research studies published in this issue aim to move beyond category-based descriptions of mathematics teacher educator knowledge and explore the broader complexity of expertise as blend of knowing, doing, believing, and being. In doing so they capture the dynamic and interconnected nature of expertise. In this commentary article, I reflect on what we can learn from these studies about the complexity of MTE expertise and use these insights to suggest directions for future research.