The CALIDIE DTU focuses on the fundamental question of how learners’ linguistic repertoires interact with their learning. It investigates in particular how multilingualism – conceptualized as the interplay of multiple linguistic repertoires (mother tongue(s), language(s) of schooling, foreign, second languages) – affects learning practices and processes, and how multilingualism can be capitalized on and transformed into a resource for educational success and social well-being. CALIDIE complements and enriches existing research by investigating this central question in (a) a specifically rich research location, (b) from a ‘cross-curricular and vertical perspective’ and (c) using an ‘interdisciplinary approach’.(a) CALIDIE takes advantage of the particular educational context of Luxembourg, a society that has not only an officially trilingual school system (Luxembourgish, French, German plus English) but also a very high number of migrants, and thus a multitude of languages that are spoken in everyday life. In this situation and as a consequence of educational policy, the vast majority of students learn in a second and third language, a context that offers unique opportunities for providing innovative insights into the research question.(b) CALIDIE contributes to building a unique corpus by investigating the fundamental relationship between multilingualism, learning, and educational success, both horizontally across curricula and vertically through education systems, including the crucial points of transition learners experience as they progress.(c) The overarching research focus of CALIDIE will be investigated from different disciplinary perspectives and by using different methodological approaches. Creating cooperation among researchers from linguistics, pedagogy, and psychology, this DTU will break new ground in providing a multi-perspectival understanding of how multilingualism and learning interact. Adopting such a conceptual and organizational frame, which innovates in terms of focus, location and approach, we aim to provide a robust base for imagining more equitable and effective educational practices and policies in Luxembourg and any other multilingual location. We want our DTU to impact on the development of multilingual pedagogies that are sensitive to multilingual learner repertoires, and take into account more complex pathways of learning.