Over historical time languages change at every level of structure: vocabulary, phonology, morphology and syntax. 1 How and why such change occurs are the key questions addressed by the discipline of historical linguistics. From the perspective of modern generative grammar, language change is narrowly constrained by the requirement that all languages conform to the specifications of the human language faculty; but the fact of language change, like the brute fact of the structural diversity of the world's languages, marks a limit to the biological specification of language. Just how wide a range of variation biology allows is perhaps the major open question of theoretical linguistics; but whatever that range may be, it is the field on which historical developments play themselves out. The necessity for a richly specified Universal Grammar follows from the logical problem of language acquisition, so that the synchronic linguist considers as candidate analyses only learnable ones couched in theories that specify clearly what is to be learned and what is built in. The modern study of syntactic change, the topic of this article, 2 is also often couched in terms of learning; but, as we will see, the study of diachrony adds complexities of its own. 1 Most of what I know about diachronic syntax, I have learned from years of discussion with my collaborators and colleagues in the field. Thanks for this ongoing dialog go first to my students and collaborators, especially Susan Pintzuk, Beatrice Santorini, and Ann Taylor, my collaborator on the Penn-Helsinki Parsed Corpus of Middle English.