This chapter outlines the complex historical processes that have shaped the present-day distribution of communities and ethnic groups across mainland Southeast Asia. The chapter first introduces some general information about the region, the languages, and some conceptual preliminaries for the book. A historical account then traces developments since prehistory through the emergence of early states in the first millennium of the Common Era, followed by the rise of new states associated with migrations from the north, into the modern colonial and postcolonial era. The chapter finishes with a discussion of the area’s politically dominant languages and a survey of the state of the art in linguistic research.