Childhood bilingualism may develop toward a steady state of balanced competence in 2 languages or toward an imbalanced competence in which one of the child's languages begins to undergo attrition or early stabilization. In child second language learning an analogous distinction is often drawn between additive and subtractive bilingualism. This review of the research focuses on the latter, the developmental shift toward 1 primary/dominant language. The studies of first language attrition reviewed in this article also offer a new way of looking at related questions in the field of language learning: critical period effects in first and second language, access to Universal Grammar in second language learning, and the componential nature of language ability.