The ability to reflect on teaching is crucially important for the professionalization of trainee teachers. The fostering of reflection processes -- by means of the systematic analysis of one's own and others' teaching from subject-specific, methodological, and educational science perspectives -- plays a central role in a continuing and consecutive two-semester teacher training seminar concept at the University of Mainz. This paper explores the different effects of reflection on teaching on the basis of a quasi-experimental pre-post test design. Additionally, it shows how systematic reflection on teaching affects the students' subjective experience of competence as well as the extent the seminar enables them to identify themes relevant for reflection. For example, the results of the comparison between two student groups working with different methodological approaches reveal that there are specific effects of opportunities for reflection on teaching. It was found that the students in both groups rated the relevance of self-reflection for their own later work as very high and felt much more confident in dealing with external video material. Moreover, the students in the intervention group appear to be increasingly capable of analysing teaching and learning processes in a more nuanced and systematic way based on theory and criteria; they are also more skilled at identifying reflection-relevant situations from various perspectives. As a consequence, it becomes clear that reflection on one's own and others' teacher actions must be integrated more strongly and consequently into teacher training in order to advance the development of one's own professionalisation and to make the best possible use of the associated potential.