In times of globalization and cultural openness, policies increasingly promote multilingualism as a strong social and economic asset. One way to foster multilingualism in education is Content and Language Integrated Learning (CLIL), a didactic method in which school subjects are taught in a different target language than the mainstream school language. To this day we only have an incomplete and fragmented view on how CLIL differs from non-CLIL education and on how it impacts second/foreign language acquisition. On the basis of a large-scale longitudinal study, the research project aims to gain insight into the linguistic, cognitive and educational aspects of CLIL and to understand how the interplay between those three perspectives may underlie L2 acquisition processes. To this end, the project will concentrate on French-speaking CLIL and non-CLIL learners (control group) having Dutch or English as target language. Data will be collected at different times in the last two years of primary and secondary school education. This interdisciplinary study intends to make a strong empirical and theoretical contribution to the ongoing international scientific debates on multilingualism in general and CLIL in particular.