This chapter first reviews early methodological and theoretical debates regarding the nature of variation above the level of phonology. These debates include whether or not the notion of the linguistic variable can be legitimately extended to morphosyntactic variation; the nature of the relationship between quantitative data and the statistical results based on them and linguistic competence; and what role linguistic introspection should play. The discussion deals with current trends in modeling morphosyntactic variation, or, put differently, with the emerging field of socio-syntax. There are two main responses to the question of where morphosyntactic variation comes from. One perspective involves the postulation of multiple grammars, most prominently in Kroch’s Competing Grammars model. The other relies on the Minimalist operations of feature interpretation and feature checking.