Due to the increasing importance of digitalization in our society, there are new demands on school, teaching and thus on teachers. Nowadays digital competences belong to the key competences of teachers. The acquisition of digital competences should already be developed in university education of teachers. In this regard, we refer to a university seminar aiming to support professional competences of prospective teachers in digital mathematical teaching and learning contexts. The seminar is based on learning mathematics with the digital assessment system STACK, reflecting the use of STACK, designing learning environments with STACK and, finally, reflecting school students' use of STACK. We collected data of prospective teachers' development of digital competences through interviews. First results show that prospective teachers' competences including knowledge, beliefs and motivation can be developed and differentiated.