Analyzing three waves of data from the Children of Immigrants Longitudinal Survey, this article explores the process of language assimilation among second-generation Latinos. Although previous studies have focused on the shift from mother tongue to English across immigrant generations, few have examined change in language proficiency over time within the second generation. Results from a series of growth-curve models suggest three findings. First, both English and Spanish proficiency increased over time. Second, significant differences exist across Latino ethnic groups, with Mexicans being least proficient in English and most proficient in Spanish. Third, use of Spanish at home and in school has no effect on English acquisition, but significantly promotes Spanish retention. Overall, these findings lend support for the emergence of a new context for bilingualism.