Teaching multilingual learning groups is an important part of the professional activity of primary teachers in Germany, placing high demands on their occupational professionalism. The aim of this article is to analyse the effects of educational and occupational influencing factors on the beliefs of teachers regarding multilingualism as a facet of pedagogical professionalism. The data is based on the 'BLUME-study', in which primary teachers (N = 123) were asked about their beliefs regarding multilingualism. A linear regression model was used to analyse the extent to which the teachers' beliefs are influenced: (a) by participating in learning opportunities on 'German as a Second Language (GSL)' in teacher training; (b) through experiences with multilingual students in teaching; (c) by participating in training on multilingualism in professional practice; and (d) by undertaking language-sensitive school programmes. Therefore, we analyse influencing factors drawn from both the university phase and the occupational phase of teacher professionalisation. The results show that the use of learning opportunities on the topic of 'GSL' in pre-service training and in in-service training as well as the positive evaluation of contact experiences in teaching lead to supportive beliefs towards multilingualism.