This paper discusses the role of sociolinguistics in the development of historical linguistics in general, and then examines the particular importance that sociolinguistics has for the linguistic history of Spain and Spanish America. Particular attention is given to the relevance of accommodation theory (Giles, 1980), dialect contact theory (Trudgill, 1986), and social network theory (Milroy & Milroy, 1985) to an understanding of the way that Spanish developed in the Middle Ages and the early modern period. A series of koineizations took place in Central and Southern Spain, in the Balkans, and in the Americas, resulting from the processes of social and dialect mixing which the Reconquest of Islamic Spain, the expulsion of the Peninsular Jews, and the settlement of the American colonies entailed. The main conclusion from this approach to the history of Spanish is that linguistic history should not be regarded as a linear process, but one which is discontinuous, full of blind alleys, hiccups, and new starts. Sociolinguistics has taught language historians, including those working with Spanish, that it is not true to say that ‘language changes’; what happens is that speakers change language.