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.