'This expert study weaves together inquiries in theoretical and historical linguistics and acquisition of language, providing new ideas and insights about the nature of language and about the emergence of languages through history and in the individual. Lucidly written and accessible to a general audience, its original perspectives and acute analyses of wide-ranging empirical materials also provide fascinating challenges to those working at the borders of research in the many areas that Lightfoot engages. An impressive and very valuable contribution.' Noam Chomsky, Massachusetts Institute of Technology