Axel Schulz examines the diagnostic skills and support competencies of teachers regarding specific issues in learning to calculate. He conducts interviews and analyzes teachers' reactions to real student documents and videos to determine the practical knowledge related to the two main symptoms of calculation disorders, namely entrenched counting strategies and a lack of understanding of place value. The aim of the work is to identify and describe the skills that enable teachers to recognize problems and develop appropriate support measures. The study thus provides suggestions for the further development of teacher training and continuing education.