Mainland Southeast Asia provides a dramatic demonstration of the areal phenomenon in linguistics: When languages are spoken historically in the same location they often show significant parallels in the organization of a wide range of structural domains, whether the languages descend from the same historical source. The effects of areal diffusion raise fundamental questions for the traditional essentialist vision of languages as entities with offspring that diverge, with shared innovations marking divergent branches and internal processes of evolution accounting for diversity among modern languages. Recent theoretical and empirical research on linguistic diversity, language change, and social diffusion of innovation argues for a unit-based approach to language change and relatedness, where the units of analysis are individual speakers and individual linguistic items. This review begins with discussion of the language situation in Mainland Southeast Asia, where the language “genealogies” have been dramatically permeated by socio-historical contact, then explores theoretical and methodological implications for research on language both generally and in its areal context.