More than 25% of Sweden’s preschool children are growing up both with Swedish and a minority home language. Whilst we know that there is a lot of individual variation in how these two languages develop over time, we lack systematic longitudinal studies. Will home language proficiency improve, stagnate, or even decline? Will the child catch up with monolingual Swedish age peers? And what exactly contributes to successful child bilingualism in Sweden? To tackle these questions, 40 Arabic/Swedish and Turkish/Swedish children are studied from age 4 to 6 and age 6 to 8. Arabic and Turkish are in focus because they are well represented in Sweden and spoken by families with a wide variety of backgrounds. As vocabulary and storytelling skills are known to develop strongly from preschool to school age, the project investigates children’s performance on lexical and narrative tasks in both languages. Children can thus be compared with themselves over time, within group and across groups. The results are also compared with an earlier, cross-sectional study of 200 children. Detailed background information about the child’s language environment in the home and (pre)school, about family language policy and language- and literacy-related activities serve to filter out factors that influence bilingualism outcomes. The project thus lays the foundation for evidence-based advice on how best to foster successful active bilingualism in children speaking a heritage/minority language in Sweden.