The main goal of this longitudinal project is to identify patterns and outcomes of vocabulary learning that separate bilingual preschool children at risk for persistent language impairment (LI) from their typically-developing (TD) age-matched peers. It is well-documented that there is an urgent need to provide clinical services for bilingual children from many immigrant communities. However, because of the lack of valid clinical language measures available for bilingual children, clinicians oftentimes are not able to accurately identify bilingual children with language impairment. In order to clinically serve these bilingual children, scientific knowledge of bilingual children's language development is clinically needed. Of key interest in this project are the ongoing interactions between the available input in each language and the learners' internal cognitive-linguistic processing systems. In order to capture the complex dynamical interactions between the external input and the internal systems, this project uses research methods that are consistent with bilingual children's dual language development and their language environment in two studies. The first study investigates how bilingual children with LI and their TD peers learn individual new words in each language over time. The focus is on the word-learning growth curves of each language. The second study examines the trajectories of children's receptive and expressive vocabulary knowledge controlling for the amount of Cantonese and English used in their home and preschool environments. The completion of this project can potentially improve the clinical procedures for identifying language impairment in the highly variable population of bilingual children. Importantly, the results from this proposed study can be compared with the findings from other bilingual children in similar communities such as the Spanish-English communities and the Vietnamese-English communities. In addition, the typological differences between Cantonese and English allow us to examine the underlying factors that contribute to bilingual children's language learning and breakdown.