These proceedings are available as a complete volume online on the ERME website and each individual text is also available on the HAL open archive, where it can be found through keywords, title or author name. This has been the practice since CERME9, to increase the visibility of the huge work done in CERME conferences. This volume begins with texts corresponding to the three plenary activities of CERME12: the plenary lecture by Susanne Prediger (Germany) on “Enhancing language for developing conceptual understanding: A research journey connecting different research approaches”; the plenary lecture by Jeppe Skott (Sweden / Norway) on “Conceptualizing individual-context relationships in teaching: Developments in research on teachers’ knowledge, beliefs and identity”; and finally the panel discussion “Big Questions in Mathematics Education”. This panel discussion was led by Anna Baccaglini-Frank (Italy), Ingi Højsted (Faroe Islands) and Janka Medova (Slovakia), chaired by Michiel Veldhuis (The Netherlands) and moderated by Eirini Geraniou (UK/Greece). The two plenary speakers, Susanne Prediger and Jeppe Skott, each gave a response to the panel discussion. After the plenaries, the reader will find 27 chapters corresponding to the work done in the TWGs of CERME12 (with combined introductions from all the split TWGs). These chapters follow a similar structure: they start with an introduction; then the long paper contributions (8-page papers) and the short poster contributions (2 pages) are presented – in alphabetical order by first author’s name. There are two kinds of introductions to the TWGs, according to the team’s choice: short introductions (4 pages) presenting the contributions; or long introductions (8 pages), which propose, in addition, an analysis of the current research on the theme of the TWG, and perspectives for the future. TWGs 04, 06, 07, 09, 14, 15, 17, 18, 19, 22, 25 and 26 have chosen this form of long introduction. The publication of these proceedings is the result of a collaborative work, involving the CERME12 IPC, the TWG leaders and co-leaders, the LOC chair and co-chair and the wider team at Bozen-Bolzano. Particular thanks are due to Katrin Lambacher. We warmly thank all these people for their involvement, and hope that this volume will contribute to the development of mathematics education research in Europe and beyond.