The Principles-and-Parameters approach to linguistic theory has triggered an enormous amount of work in comparative syntax over the last decade or so. A natural consequence of the growth in synchronic comparative work has been a renewed interest in questions of diachronic syntax, and this collection testifies to that trend. These papers focus on questions of clause structure which have become a central theme of theoretical work since the pioneering work in the late 1980s by Chomsky, Pollock, and others. The languages studied by an international roster of contributors include all the major Romance and Germanic languages. This volume is of central importance for anyone working in theoretical, comparative, or historical syntax.