Teachers’ beliefs about teaching and learning of mathematics are said to be crucial if thinking about supporting students’ learning at school. That’s why several empirical studies discuss the influence of teachers’ beliefs as a part of teach-ers’ competence on the quality of teaching mathe-matics within the last years. Nevertheless, it’s known little about how teachers’ beliefs about teaching and learning (mathematics) can be changed successfully. Therefore, within a teacher training study as part of the research-project Co2CA the effect of teacher trainings on teachers’ beliefs has been observed empirically. Over a period of overall 10 weeks N = 30 mathematics teachers took part in teacher trainings dealing with formative assessment if teaching mathematical modelling (ex-perimental group A; EG A). Another N= 37 teach-ers participated in teacher trainings dealing with general didactical ideas concerning mathematical modelling and mathematical problem solving (ex-perimental group B; EG B). Next to assessing teachers’ pedagogical content knowledge about central ideas of formative assessment (PCK-FA) teachers’ beliefs about formative and summative assessment have been asked for several times. Ana-lyzing the influence of these teacher trainings on teachers’ pedagogical content knowledge and teachers’ beliefs it can be pointed out: While teach-ers out of EG A outperform their counterparts out of EG B within the PCK-FA-test at the end of the teacher trainings, teachers’ beliefs about assess-ment in classroom within EG A do not change with-in 10 weeks of teacher training. Furthermore, dis-cussing teachers’ beliefs by comparing teachers out of EG A with teachers out of EG B, only little differ-ences in teachers’ beliefs about assessment in class-room do exist.