Using cohabitation and marriage histories collected in 1985 from 23-year-old women and men, this study investigates the process of union formation, considering transitions from single life into cohabitation and marriage. The outcomes of cohabitation are also considered-both the dissolution of unions and the transformation of cohabiting unions into marriage. These data indicate that large proportions of men and women experience cohabitation fairly early in the life course. At the same time, many cohabiting unions are dissolved fairly quickly and numerous others are soon transformed into marriages. Thus even though cohabitation will be experienced by many, most people will continue to spend substantially more time in marital unions than in cohabiting unions.