This volume contains the proceedings of MADIF 12, the twelfth Swedish mathematics education research seminar, held in Växjö, January 14–15, 2020. The theme for this seminar was Sustainable mathematics education in a digi- talized world. The MADIF seminars are organised by the Swedish society for research in mathematics education (SMDF). MADIF aims to enhance the opportunities for discussion of research and exchange of perspectives, amongst junior researchers and between junior and senior researchers in the field. [...]