The last two decades have seen a wealth of research on the syntax of heritage languages, i.e., languages acquired in a naturalistic way in addition to the majority language of the society. So far, the heritage language field has mostly been confined to case studies of individual language dyads. My project will move beyond that and apply a comparative angle by studying grammatical constructions across various combinations of languages. In particular, the project will apply a cross-linguistic perspective to the syntax of definiteness marking in heritage languages. Bringing together a diverse group of world-leading scholars with complementary expertise, the project aims to change the research front on definiteness marking in multilingual grammars by developing a theoretical syntactic model of definiteness marking that is cross-linguistically valid.