The study of child L2 acquisition has much to offer the field of language acquisition. This is because the L2 child (whose exposure to the Target Language starts between the ages of, say, 4 and 7) shares certain characteristics of the L1-acquiring child and other characteristics of the L2-acquiring adult. Research on child L2 acquisition thus has the potential to inform our understanding of both native language acquisition and adult non-native language acquisition. In regard to the former, the L2-acquiring child, while cognitively more mature than the L1-acquiring child, is nevertheless a child. On the assumption that UG guides both cases, comparisons between L1 development and child L2 development can therefore address the validity of maturation-based explanations of L1 acquisition, be they general cognitive or specific to language. Maturational explanations of L1 development are compatible with L1-L2 child differences but incompatible with L1-L2 child similarities. As for nonnative language, the L2 child, like the L2 adult, has a (more or less) complete grammar at the outset of acquisition. On the assumption that L1 transfer occurs in both cases, comparisons between the L2 child and the L2 adult can therefore address the extent to which UG constrains adult L2 acquisition. Theories of UG-constrained adult L2 acquisition are supported by L2 child-adult similarities but (potentially) refuted by L2 child-adult differences. Finally, three-way (developmental) comparisons of the L1 child, the L2 child and the L2 adult should be able to pinpoint principled reasons for the bifurcation in ultimate attainment between, on the one hand, child (L1 or L2) acquirers and, on the other, adult L2 acquirers.