Anna-Maria Aldorf investigates how teachers cooperate on topics relevant to professional development. The collaboration among teachers and their professional training has proven to be a significant characteristic of organizational quality, aimed at improving classroom instruction, the teaching and learning culture, as well as student learning outcomes. In the context of the EU research project PRIMAS, the author explores the relationship between these collaborations and the effectiveness of teacher training, providing an overview of cooperative approaches, such as encouraging communal reflection processes during professional development events that support teachers.