This chapter summarizes key findings on language assimilation and discusses the central implications for the education of immigrant students. It begins with a description of language shift, which covers the present state of knowledge on changes in language proficiency and use over time and across generations. The focus then turns to the conditions of language acquisition and language use. Both sections provide an overview of the central theoretical arguments, followed by a summary of the main empirical findings. The ramifications of destination language fluency and use for the education of immigrant students are then discussed.