Research on the mathematics teacher educator (MTE) is an emerging field that is important to understand effective mathematics teacher education in a rapidly changing world. This edited volume addresses the expertise of MTEs —those who support mathematics teachers’ learning and development at various stages of their careers. It positions MTE expertise as extending beyond category-based knowledge, offering new ways of conceptualizing and examining expertise. Part 1 presents global studies that examine MTE expertise in practice. Part 2 highlights innovative methodologies for investigating MTE learning and development. Part 3 features reflective essays by distinguished MTEs, sharing personal insights, historical accounts, and pivotal moments in their professional journeys. This volume is a valuable resource for both new and experienced MTEs seeking to deepen their practice and research.