This chapter provides an introduction to the Basque language, its dialects, sources for linguistic data on the language, the relationship of Basque to Aquitanian, and a brief history of scholarship on the language. The chapter also introduces the central goal of Part I of the book: to reconstruct the sound system and root lexicon of Proto-Basque, the mother language of all Euskarian languages, as far back in time as evidence permits. Within the field of historical linguistics, Basque is viewed as an isolate, meaning that there is no genetic relationship between Euskarian languages and any other known languages, living or extinct. As a consequence, the comparative method of historical linguistics, contributes to the reconstruction of Proto-Basque only at the level of dialects and comparison of Basque to Aquitanian. Comparative work must be combined with internal reconstruction. The method of internal reconstruction, like the comparative method, is based on the Neogrammarian hypothesis that sound change is regular. In this chapter, the most widely accepted proposals for Proto-Basque phonology that make use of dialect comparison and internal reconstruction are reviewed, with a focus on Michelena’s collected works and Lakarra’s more recent studies of root structure.