The study of comparative grammar has long been a concern of linguistic theory. To the extent that, by studying the aspects of grammar which vary, we might arrive at an idea of what does not vary, this study can be seen as one way of studying universals of grammar. Although it has antecedents in the Middle Ages, comparative grammar was not systematically studied until the nineteenth century, and then purely from a historical perspective. In the past forty years, however, two important approaches have emerged: Greenbergian language typology and the Chomskyan programme based on the idea of the interaction of the principles and parameters of universal grammar. In recent years, these two approaches have to a degree converged. Our notion of how grammatical systems vary and our ability to provide detailed, sophisticated analyses of this variation across a range of languages and grammatical phenomena is probably greater than it has been at any time in the past. Concentrating on principles-and-parameters theory, this new Routledge Major Work presents a general, detailed and critical overview of what has been achieved. Aside from the first and last volumes, each one is devoted to a particular aspect of grammatical variation which has been identified as underlying important differences among languages. The first volume presents some of the most important work prior to the formulation of the principles-and-parameters approach in approximately 1980, including Greenberg's seminal early paper on language typology, while the last volume, in addition to considering further aspects of variation, briefly illustrates how the principles-and-parameters approach has been applied to first-language acquisition and syntactic change. With comprehensive introductions to each volume, newly written by the editor, which place the collected material in its historical and intellectual context, Comparative Grammar is an essential work of reference and is destined to be valued by linguistics scholars and students as a vital research resource.