One could focus on many different aspects of improving the quality of mathematics teaching. For a better understanding of children's mathematical learning processes or teaching and learning in general, reflection on and analysis of concrete classroom situations are of major importance. On the basis of experiences gained in a collaborative research project with elementary school teachers, several ideas about a professional reflection on one's own instruction activities are explained. The paper focuses on joint reflection between teachers and researchers on the participating teacher's own classroom interaction by means of concrete examples. It becomes clear that changes of one's own interaction behavior will take place only in the long-term. Nevertheless such a joint professional reflection should be an essential component of teachers' professional knowledge in a natural way.