This paper synthesizes research on the demographic correlates and consequences of unmarried, heterosexual cohabitation in the United States. First, we place cohabitation in the context of recent demographic trends in union formation and dissolution. Second, we consider the implications of cohabitation for child well-being. Third, we review population subgroup variation in the role of cohabitation in family patterns, focusing on social class and race and ethnicity. Finally, we discuss how and why unmarried cohabitation is implicated in recent dialogues about family policy.