PROJECT SUMMARY/ABSTRACT At the onset of English-dominant schooling, bilinguals with developmental language disorders (DLD) generally experience a progressive decline in home language use. Yet, theories posit that a bilingual’s two languages interact in ways that enhance children’s developing language faculty (Hernandez, 2018). To advance new perspectives on neurodevelopmental disorders of cognitive function and inform clinical approaches for linguistically-diverse learners, we seek to understand language development in bilingual children with DLD. Our objective is to uncover how bilingualism impacts language development in children with DLD. Wang et al. (2021) bridge the Interactive Specialization (Johnson, 2011) and Dual Stream (Poeppel, 2012) frameworks to conceptualize language development as an interaction between neurocognitive systems competing for language subfunctions. To examine the effects of bilingualism on DLD, we advance this perspective by linking it to the view that bilingual experiences influence extrinsic brain-behavior and intrinsic neural interactions (Hernandez, 2018). We thus hypothesize that added interactivity and competition of two languages will be positively associated with cortical specialization for language function in bilinguals with DLD. To test this hypothesis, we focus on syntactic and semantic competences disproportionately affected in DLD. We use behavioral and functional Near Infrared Spectroscopy (fNIRS) neuroimaging measures, testing Spanish-English bilinguals ages 7-9 (N=400, 60 with DLD), in two languages. Sentence judgment and narrative comprehension tasks are used to uncover cortical specificity along the dorsal and ventral language neurocircuits. Aim 1 is to determine language-specific effects of bilingualism on the finetuning of cortical specialization for language function in DLD. Aim 2 is to uncover the global-system effects of bilingualism on cortical specialization for language in DLD. To achieve these aims, we examine the relation between children’s syntactic and semantic competences in each language, the relative balance in dual language experiences, and their interaction with children’s emerging neural architecture for language. This principled approach will provide an empirical basis to (1) uncover mechanisms that support the advancement of English proficiency in bilinguals with DLD; (2) advance theories of DLD through neuro-cognitive evidence on language competences most affected in DLD; (3) inform intervention by illuminating individual differences in the relations between language experiences, proficiency, and brain development. This evidence will reveal mechanisms by which language experiences and proficiency interact with DLD, providing theoretically rich and clinically significant information.