The objective of this paper is to identify the incentives and barriers to marriage among cohabiting women, especially disadvantaged mothers who are targets of welfare reform. We use the newly released cohabitation data from the National Longitudinal Survey of Youth (1979-2000), which tracks the partners of cohabiting women across survey waves. Our results support several conclusions. First, cohabiting unions are short-lived--about one-half end within one year, and over 90% end by the fifth year. Unlike most previous research, our results show that most cohabiting unions end by dissolution of the relationship rather than by marriage. Second, transitions to marriage are especially unlikely among poor women; less than one-third marry within five years. Cohabitation among poor women is more likely than that among nonpoor women to be a long-term alternative or substitute for traditional marriage. Third, our multinomial analysis of transitions from cohabitation into marriage or dissolution highlights the salience of economically disadvantaged family backgrounds, cohabitation and fertility histories, women's economic resources, and partner characteristics. These results are interpreted in a policy environment that increasingly views marriage as an economic panacea for low-income women and their children.