This experience refers both to observations of lessons given by the mentor as well as to teaching to 'own' classes. Observations of classes often promote knowledge of students' conceptions and difficulties. Observation of lessons given by the mentor often lead to a change of PCK of teaching strategies towards the mentor's PCK. Only after a period of own teaching activities, the 'imitated' PCK can change toward more authentic knowledge. The teaching activities also provide student teachers the opportunity to use their SMK constantly and in an intensive way. Nevertheless, this process runs quite slowly because PCK change is hindered by the complexity of the classroom situation. In the beginning of their teaching practice, many student teachers struggle with difficulties in classroom management such as enforcement of the class rules, control of the lesson time schedule, and care of an efficient organization of group work. Initially, they focus more on teaching strategies for 'survival' than on teaching strategies for understanding specific topics. In other words, their self-concern is more dominant than concerns about content-oriented teaching and about student learning (Kagan, 1992) Fortunately, when teaching experiences increase, teachers' concerns shift towards task-concern and student-concern. Many student teachers integrate subject matter knowledge and general pedagogical knowledge more and more. Finally, I will point out that the reverse influence is also possible. The preparation and teaching of lessons will force student teachers to focus on their knowledge of subject matter, for instance, when thinking about explanations of difficult science concepts and to enhance this knowledge if necessary. This process is reflected in the well-known proverb: you often do not understand a topic well until you have to teach it to somebody