This book applies recent theoretical insights to trace the development of Castilian and Latin American Spanish from the Middle Ages onwards, through processes of repeated dialect mixing both within the Iberian Peninsula and in the New World. The author contends that it was this frequent mixing which caused Castilian to evolve more rapidly than other varieties of Hispano-Romance, and which rendered Spanish particularly subject to levelling of its linguistic irregularities and to simplification of language extended into and across the Americas. These processes are viewed in the context not only of the Hispano-Romance continuum (which includes Galician, Portuguese and Catalan) but also of the New World varieties of Spanish. The book emphasises the subtlety and seamlessness of language variation, both geographical and social, and the impossiblity of defining strict boundaries between varieties. Its conclusions will be relevant both to Hispanist and to historical sociolinguistis more generally.