This fourth international handbook discusses developments not recognized or dealt with fully in the first three Springer Mathematics Education handbooks and tackles controversial issues in the field. After starting with a provocative introductory chapter which asks whether controversy is a healthy feature of international mathematics education, the four following sections cover: (a) mathematics education in Asia; (b) the roles of theory in research and practice; (c) equity and social justice; and (d) curriculum and change. These themes are taken up in 28 chapters by 60 authoritative authors from all continents. Each of the four sections is structured on the basis of past, present, and future aspects. Like the first three mathematics education handbooks, this handbook provides a very valuable resource for teachers, mathematics education practitioners and researchers, education policy makers, and mathematicians, as well as graduate and undergraduate students.