This book presents a new reconstruction of Proto-Basque, the mother language of modern Basque varieties, historical Basque, and Aquitanian, grounded in traditional methods of historical linguistics.Building on a long tradition of Basque scholarship, the comparative method and internal reconstruction, informed by the phonetic bases of sound change and phonological typology, are used to explain previously underappreciated alternations and asymmetries in Basque sound patterns, resulting in a radically new view of the proto-language.The comparative method is then used to compare this new Proto-Basque with Proto-Indo-European, revealing regular sound correspondences in basic vocabulary and grammatical formatives.Evaluation of these results supports a distant genetic relationship between Proto-Basque and Proto-Indo-European, and offers new insights into specific linguistic properties of these two ancient languages.This comprehensive volume, which includes a detailed appendix including Proto-Basque/Proto-Indo-European cognate sets, will be of general interest to linguists, archeologists, historians, and geneticists, and of particular interest to scholars in historical linguistics, phonetics and phonology, language change, and Basque and Indo-European studies. Errata for the book can be found at: https://julietteblevins.ws.gc.cuny.edu/proto-basque/