Project Summary Language ability before entry to kindergarten is among the best predictors of later success in school, yet the evidence base to support oral language development in Dual Language Learners (DLLs) across the full range of language abilities is lacking. To address this known gap, this research investigates the specific contributions of (a) the quantity of language input in the home environment, (b) the quality of language learning opportunities during interactions with caregivers, and (c) language learning processes that support children's bilingual language development. The central hypothesis is that a multidimensional approach to bilingual development that includes child, dyadic, and family factors will more strongly predict learning outcomes. Critically, this work will include children across the full continuum of language development – from typical skills in both English and Spanish to children with weak skills in both languages (i.e., language delayed). Under Aim 1, we test toddlers' ability to make rapid inferences about the meaning of a novel word based on limited exposure (i.e., fast- mapping) in English and Spanish, asking how word learning relates to vocabulary and grammatical development in children with varying levels of language skill. Under Aim 2, we examine how the quantity of dual language input and the quality of language learning opportunities during caregiver-child interactions explain variability in word learning performance and subsequent bilingual language outcomes one year later. To meet these aims, DLLs and their caregivers will be studied longitudinally across a well-known period of rapid lexical acquisition in toddlerhood. This work is significant because it will identify the contribution of child, dyadic, and family variables for bilingual language development and examines these factors across the full continuum of language skills. The proposed work is innovative because it (a) tests bilingual word learning skills by evaluating fast-mapping in both English and Spanish; and (b) takes an interdisciplinary and multidimensional approach to language development including the child's language learning skills in combination with the learning environment. Data will provide new insight into sources of variability that influence early dual language trajectories, and point to strategies for supporting long-term language success.