Drawing on a short analysis of a classroom episode, we reflect on the teacher’s actions and their relationship to his/her didactical knowledge, namely in its dimensions of knowledge of mathematics and knowledge of instructional processes. Focusing on these dimensions, we discuss the answers of some future and practicing teachers to a written assignment based on that episode. Anchored in the notion of didactical knowledge, we raise some issues regarding teacher education programs and their adequacy to comply with current demands of mathematics teaching.