The article considers the phenomenon of multicultural society. The author shows that the term multicultural(ism) is being used in three different contexts: one is political referring to policies and institutions; another is empirical, describing societies; the third is connected with political and social theory and philosophy. In all three contexts, both proponents and opponents use the word. The article discusses four major types of multicultural societies, each with a specific origin and dynamic: pre-modern empires, the New World settlements, colonial and ex-colonial societies, and post-national multiculturalism in recent North America, Oceania, and Western Europe. The author considers that the new self-conscious multiculturalism came out of a new diversity and wave of immigration, as well as new assertiveness and a new understanding of the indigenous peoples of the New Worlds and of American Blacks. Another source was rooted in the new cultural movements, such as feminism and, in the US, movement for gay liberation. Canada and Australia adopted explicitly and prominently multicultural policies and institutions, whereas the US and the Western European record is more patchy and implicit. The author argues that multiculturalism and its equivalents have been and are politically controversial, and have given rise to various uniculturalist counter-movements. The article shows that multicultural societies and movements raise a number of challenges to inherited Western political philosophy, social theory, and political ideologies. They concern the significance of identity and the functioning of cultures, the proper construction of a polity, the tasks of conservation and the agencies of change.  Some of these have already spawned a number of treatises and debates reviewed in the article.