Abstract Early exposure to two languages is widely thought to guarantee successful bilingual development. Contradicting that belief, children in bilingual immigrant families who grow up hearing a heritage language and a majority language from birth often reach school age with low levels of skill in both languages. This outcome cannot be explained fully by influences of socioeconomic status. In this article, I summarize research that helps explain the trajectories of observed dual language growth among children in immigrant families in terms of the amount and quality of their language exposure as well as their own language use.