There is not only a consensus that modelling is a big idea for mathematics as a scientific discipline, but also that modelling should have an impact on the teaching of mathematics. Consequently, teachers should be aware of this big idea and know how modelling relates to a variety of curricular contents. However, there is still a need of broadening especially quantitative empirical research into views of pre-service and in-service teachers related to modelling as a big idea. Hence, the research interest of this study concentrates on views of about 40 Austrian pre-service teachers and more than 20 in-service teachers about the significance of modelling as a big idea and on corresponding professional knowledge. For this purpose, we focus on perceptions of the role of modelling as a big idea, and report about results concerning the professional knowledge of the pre-service and in-service teachers. The methodological approach is based on a questionnaire and test instrument containing items in open and standardised formats. The results show differences among the sub-samples, respectively, and suggest needs of professional development, which can be targeted on the specific deficits, e.g. by focusing on connecting elements of subject matter through the idea of modelling.