Since the 1960s, a variety of new ways of addressing the challenges of diversity in American society have coalesced around the term "multiculturalism." In this article, we impose some clarity on the theoretical debates that surround divergent visions of difference. Rethinking multiculturalism from a sociological point of view, we propose a model that distinguishes between the social (associational) and cultural (moral) bases for social cohesion in the context of diversity. The framework allows us to identify three distinct types of multiculturalism and situate them in relation to assimilationism, the traditional American response to difference. We discuss the sociological parameters and characteristics of each of these forms, attending to the strength of social boundaries as well as to the source of social ties. We then use our model to clarify a number of conceptual tensions in the existing scholarly literature and offer some observations about the politics of recognition and redistribution, and the recent revival of assimilationist thought.