This contribution presents results from a research project funded by the BMBF (Federal Ministry of Education and Research). It introduces a questionnaire on the beliefs of primary school teachers on multilingualism in North Rhine-Westphalia, Germany. Regarding disadvantages of migrant students in school success, deficits in the majority language are often identified as a determinant. This perspective originates from a monolingual habitus, which causes missing consideration of multilingualism and devaluation of minority languages in school (Gogolin 1994). Yet, the field of psycholinguistics has established the consensus that linguistic and cognitive transfer occurs when learners are encouraged to access their entire language repertoire (Cummins 2000). We hypothesize that to counteract the monolingual habitus and provoke school development, it is necessary to impart knowledge, change teachers’ beliefs and supply strategies for incorporating multilingualism. Our project thus provides intervention via in-service training for teachers and educators in four primary schools. We suspect that the monolingual habitus is reflected in teachers’ beliefs about multilingualism. To capture beliefs and quantify the success of the intervention, a pre-post-design in the form of a quantitative questionnaire is utilized. The sample consists of 160 persons. The operationalization is based on field-tested self-efficacy-items and self-developed items on multilingualism.