Parents often ask me for advice about exposing their children to two languages in the home. Typically, one of the parents speaks some language other than English and they are concerned that their linguistic decisions will have consequences for the child's development. The requests come in many forms (although e-mail has become the channel of choice) and from people with obviously different levels of background knowledge, education, and experience. The motivation for their questions is usually the same – will the child learn English and will the experience of learning two languages lead to either cognitive or linguistic confusion?

These questions are interesting because of the assumptions they reveal about the folk wisdom of childhood bilingualism. First, people intuitively believe that language learning is a fragile enterprise and can be easily disrupted. Second, they assume that languages interact, and that learning one language has implications for learning another. Finally, they expect that what happens with language can impact on the rest of cognition.

All of these assumptions are empirical questions and all of them entail theoretical controversies. Moreover, they are questions for which controlled investigation is difficult, if not intractable. Ironically, it is bilingual children who also provide the most promising forum for their examination and a means of potentially resolving the theoretical disputes. What happens to children's developing knowledge of language if they are learning two languages at the same time? How do children sort out the words and meanings from the two systems and incorporate them into thought?