At the time of the above dialogue, Alicia at age one understands English but does not produce it, while already producing recognizable words in Cantonese. Such a ‘silent period’ is a common situation in bilingual children, as discussed in chapter 3, and one indication that one of the child's languages is dominant. As her elder sister Sophie has worked out by age five, somehow Alicia will grow up to be bilingual just like her, without actually being taught. Sophie even has a ‘theory’ of how this happens, which she elaborated on another occasion: children who hear each parent speaking a different language reply in the same language, becoming bilingual as a matter of course.

This chapter presents a theoretical framework within which we embed the central issues discussed in the book. We first raise and discuss theoretical issues concerning the epistemology of bilingual first language acquisition in relation to child second language acquisition (section 2.1) and the logical problem of bilingual acquisition (section 2.2). We then discuss central research issues in bilingual acquisition including language differentiation (section 2.3), language dominance (section 2.4), cross-linguistic influence (section 2.5), input ambiguity and learnability (section 2.6) and vulnerable domains (section 2.7).