Teachers are the decisive factors in ensuring the achievement of creative and sustainable learning outcomes in mathematics and science education, in fostering young peoples' competences and enabling them to become critically thinking, responsible and active citizens. Teacher professional development (PD) is the key to effecting change in mathematics and science education throughout Europe. Whilst the relevance ofteacher PD is beyond dispute, there is still too little knowledge about how to scale-up professional development successfully and sustainably. Initiating discourse and leveraging international exchange about concepts and experiences concerning this essential topic were the main aims of the conference 'Educating the Educators - International approaches to scaling-up professional development in mathematics and science education', which took Place in December 2014 in Essen, Germany. The conference was hosted by the EU-funded project mascil mathematics and science for life), coordinated at the University of Freiburg, Germany, and the DZLM (German Centre for Mathematics Teacher Education) and brought together researchers and practitioners: teacher educators, multipliers and teachers Who are all engaged in the field of maths and science education. This volume reflects particpants' conference experiences. Further, we are pleased to provide here background information and aims of the conference issue, as well as conference conclusions and an outlook on future needs and trends. The papers relate to four conference tracks, each addressing a different approach to scaling-up professional development in maths and science education: (1) Scaling-up with multipliers in face-to-face professional development courses (2) Blended learning concepts and e-learning support (3) Disseminating and scaling-up through materials (4) Professional learning communities.