In light of the current discussion on teacher professionalism, Hannah Heinrichs explores how diagnostic competencies of (future) teachers can be effectively promoted. The author focuses on the error diagnosis competence of mathematics teacher education students and examines how the handling of student errors, their quick identification, targeted interpretation, and the development of appropriate action alternatives can be fostered already in the first phase of teacher training. She demonstrates that the error diagnosis competence of the students can be positively influenced through a university course.