Abstract Over the past five decades, unmarried cohabitation has become widely accepted—even normative—among Americans. Support for cohabitation has increased among many groups, from teenagers to elders. In fact, most first coresidential romantic unions are cohabitations rather than marriages. Yet while the majority of American adults will enter a cohabiting union, most cohabiting unions do not lead to marriage, and the shares of those engaging in serial cohabitation are rising. Whereas many young adults in the 20th century viewed cohabitation as a stepping stone to marriage, in the 21st century cohabitation increasingly serves as an intensive form of dating—at least at its inception. The pathways into marriage from cohabitation differ in important ways by social class, highlighting how union formation contributes to growing levels of inequality.