This second edition of the International Handbook of Mathematics Teacher Education builds on and extends the topics/ideas in the first edition while maintaining the themes for each of the volumes. Collectively, the authors looked back beyond and within the last 10 years to establish the state-of-the-art and continuing and new trends in mathematics teacher and mathematics teacher educator education, and looked forward regarding possible avenues for teachers, teacher educators, researchers, and policy makers to consider to enhance and/or further investigate mathematics teacher and teacher educator learning and practice, in particular. The volume editors provide introductions to each volume that highlight the subthemes used to group related chapters, which offer meaningful lenses to see important connections within and across chapters. Readers can also use these subthemes to make connections across the four volumes, which, although presented separately, include topics that have relevance across them since they are all situated in the common focus regarding mathematics teachers. Volume 3, Participants in Mathematics Teacher Education, focuses not only on prospective and practicing teachers as learners but also on school colleagues, teacher educators, researchers, and others who work to provide effective learning opportunities for teachers. The emphasis is on describing and analysing participants’ engagement in mathematics teacher education collaborations and contexts from various perspectives. Thus, as the third volume in the series, it further broadens our understanding of the development of mathematics teachers.