Tommy Dreyfus is Emeritus Professor of Mathematics Education at Tel Aviv University in Israel and one of the Founding Series Editors of the IMPACT Mathematics series. He is an Advisory Editor of Educational Studies in Mathematics and a Contributing Editor of the Journal of Mathematical Behavior. He has previously edited several books including one in Routledge’s EARLI book series. Michèle Artigue is Emeritus Professor of Mathematics Education at the University Paris Diderot – Paris 7 in France. She was recently awarded the Felix Klein medal for life-time achievement in mathematics education research by the International Commission on Mathematical Instruction, and is a Knight of the French National Order of the Legion of Honour. Despina Potari is Professor of Mathematics Education at the University of Athens in Greece, and Guest Professor of Mathematics Education at Linnaeus University in Sweden. She is also Editor-in-Chief of the Journal of Mathematics Teacher Education. Susanne Prediger is Professor of Mathematics Education and head of the Institute for Development and Research in Mathematics Education at the University of Dortmund in Germany. She has authored numerous journal articles and book chapters and is currently President of ERME, the European Society for Research in Mathematics Education. Kenneth Ruthven is Emeritus Professor of Education at the University of Cambridge in England and Guest Professor in Mathematics Education at Karlstad University in Sweden and at the University of Agder in Norway. He is an Advisory Editor of the international journals Educational Studies in Math-ematics and Research in Mathematics Education and a Fellow of the Academy of Social Sciences.