Teacher beliefs are action guiding in the classrooms. Teacher beliefs about inclusive education are thus a crucial prerequisite for its success. Therefore, those beliefs have to be addressed during the first phase of teacher training. Generally accepted concepts or operationalized definitions would be valuable guidelines for pre-service teachers and their educators. However, neither the ones nor the others are available at present. Therefore, pre-service teachers have to fall back on their own beliefs, a rather unexplored notion so far. Within the present study, pre-service teachers' beliefs about inclusive education were assessed before and after an academic seminar. During this academic seminar, participants co-taught in either multi-professional (i.e., one pre-service teacher for special educational needs and one for general education) or mono-professional (i.e., both pre-service teachers for special educational needs or both for general education) teams in inclusive classes of secondary schools. Pre-service teachers' beliefs were assessed with the help of concept-maps, which were created by the participants at two testing times. The concept-maps were analyzed employing graph-theoretical approaches as well as qualitative, summarizing content analysis methods. Results show that pre-service teachers who worked in multi-professional teams expanded their conceptualization of inclusive education to include facets like individualization and differentiation, while pre-service teachers who worked in mono-professional teams displayed no such expansion. Also, the conceptualization of pre-service teachers who worked in mono-professional teams contained a larger percentage of propositions addressing disadvantages and negative consequences of inclusive education. Therefore, it is concluded that multi-professional co-teaching during teacher training helps prepare teachers for successful inclusive education.