Multilingualism is promoted as an important social and economic asset, especially in Belgium where three official languages and four linguistic areas co-exist. One way to foster multilingualism in education is the Content and Language Integrated Learning (CLIL) pedagogical approach, in which some school subjects (e.g. history, geography, mathematics, etc.) are taught in a different language than the mainstream school language. This thesis - framed within a multidisciplinary research project on CLIL in French-speaking Belgium - focusses on the (longitudinal) acquisition of phraseological units by French-speaking secondary school learners of English and Dutch as a foreign language. It compares the acquisition of such units by CLIL and NON-CLIL learners. Phraseology refers to the large number of multiword combinations that compose the language (e.g. good at, in addition to, how are you, op reis gaan ‘go on holiday’, bang voor ‘afraid of’, zelfs als ‘even though’, etc.). Research shows that the acquisition of phraseological units remains a stumbling block for all foreign language learners, even at advanced levels. In fact, phraseology is not always subject to explicit instruction and implicit learning has its limits in instructed settings. Moreover, interferences from the mother tongue (L1) also impact the learning process. As the CLIL approach is considered as a more implicit and input-rich type of learning context, we hypothesized a better and faster acquisition of phraseological units by CLIL learners compared to their NON-CLIL peers in traditional foreign language classes. On the whole, the results of this thesis corroborate our hypotheses. A higher phraseological productive and receptive knowledge was observed in the CLIL group. The longitudinal results were however more mixed since the CLIL learners either positively progressed, but to a lesser extent than the NON-CLIL learners (in the case of English), or even displayed a decrease in their use of phraseological units (in the case of Dutch). ; Le ...