How to best promote proficiency in foreign languages, and the role of schooling in doing so, has been a matter of debate in the European Commission for decades. The MultiCat action addresses this debate by investigating the effects of two English learning contexts, English as a foreign language (EFL) and Content Language Integrated Learning (CLIL), on language development during the first year of elementary school in Catalonia, a Catalan-Spanish bilingual province in Spain. In CLIL contexts, students receive extended instruction in English at the expense of reduced hours in Catalan/Spanish. We investigate (1) the effects of the learning context on the development of the three languages and (2) how individual differences between children may influence such development. Special consideration is given to the multilingual development of heritage language children (i.e., children whose home language is not Catalan, Spanish, or English). As such, this action lies at the intersection of three main issues of interest: the development of foreign languages, the protection of minority languages, and the integration of heritage language children. To address our objectives, we will collect data on the oral language and literacy skills in English, Catalan, and Spanish from 600 first graders (ages 5-6) at two time points (at the onset of primary schooling and at the end of the first schoolyear). By determining the effects of the learning context and other sources of individual differences, the results from this action may inform policymakers’ decisions to promote one learning context over the other, and will help other stakeholders make informed decisions about multilingual practices in the school and the home. This action will combine the expertise of the Experienced Researcher, currently a postdoctoral fellow investigating individual differences in bilingual children, and the supervisor, whose research has focused on the effects of CLIL instruction on language development.