The aim of this project is to develop and evaluate the effectiveness of a mother tongue-based preschool language intervention program on the language and literacy outcomes of Portuguese language-minority children growing up in Luxembourg using a randomised controlled study (RCT) design. Our earlier work has shown that Portuguese immigrant children, who represent 22% of the school population in Luxembourg, manifest substantial language weaknesses in their first language Portuguese and their second language Luxembourgish. Furthermore, Portuguese-speaking children represent by far the most vulnerable group in Luxembourg in terms of oral language and reading proficiency in the school language German, as well as general academic progress. Research shows that students who start school with a well established first language are likely to develop strong abilities in their second language(s) because oral skills in the mother tongue lay a cognitive foundation for acquiring subsequent languages. The presented project will explore whether a theoretically grounded mother tongue-based language intervention can boost second language and literacy skills of language-minority children. The study builds on a preschool program from the UK (Nuffield Language4Reading) that has been proven to be effective in promoting oral language and early literacy skills in monolingual English-speaking children with language weaknesses. The original program will be adapted into Portuguese and adjusted for small-group tutorials. The program will run for 30 weeks across Year 1 and Year 2 of Kindergarten (C1.1. and C1.2) and will mainly focus on developing children’s vocabulary knowledge as well as their narrative, listening, and phonological skills. The project directly addresses the following research questions: (1) Can a mother tongue-based preschool language intervention improve first language skills in language-minority children? (2) Does such an intervention have knock-on effects on second language learning? (3) Will far transfer effects to early literacy skills in a second language emerge? This project will provide evidence for the effectiveness of an early mother tongue-based oral language intervention to improve the language and literacy development in language-minority children. The research will have important practical implications for educational policy in Luxembourg and other countries with high levels of learners from linguistic minorities.