Much theorizing in language change research is made without taking into account dialect data. Yet, dialects seem to be superior data to build a theory of linguistic change on, since dialects are relatively free of standardization and therefore more tolerant of variant competition in grammar. In addition, as compared to most cross-linguistic and diachronic data, dialect data are unusually high in resolution. This book shows that the study of dialect variation has indeed the potential, perhaps even the duty, to play a central role in the process of finding answers to fundamental questions of theoretical historical linguistics. It includes contributions which relate a clearly formulated theoretical question of historical linguistic interest with a well-defined, solid empirical base. The volume discusses phenomena from different domains of grammar (phonology, morphology and syntax) and a wide variety of languages and language varieties in the light of several current theoretical frameworks.