Teachers' expertise, that is teachers' content knowledge, pedagogical content knowledge and general pedagogical knowledge, is crucial when considering the determinants influencing teaching and learning. Nevertheless, it is unclear how teachers' expertise develops or whether teachers' expertise can be build up within teacher trainings. Within the research project ``Co(2)CA,'' teacher trainings concentrating on selected pedagogical and didactical ideas of teaching mathematics were conducted and evaluated using tests senstitive to the content of the teacher trainings. Assigned by interest, 30 teachers participated in teacher trainings discussing formative assessment of teaching competency-oriendet mathematics (Experimental Group A); 37 teachers participated in teacher trainings focusing on general questions about competency-oriented mathematics (Experimental Group B). Quantitative analyses of the expertise tests prove the effectiveness of the teacher trainings: Controlling for general pedagogical content knowledge, at the end of the training, teachers of Group A score significantly higher than teachers of Group B on the expertise test on formative assessment of teaching competency-oriented mathematics.