Most studies of union formation focus on short-term probabilities of marrying, cohabiting, or divorcing in the next year. In this study, we take a long-term perspective by considering joint probabilities of marrying or cohabiting by certain ages and maintaining the unions for at least 8, 12, or even 24 years. We use data for female respondents in the 1979 National Longitudinal Survey of Youth to estimate choice models for multiple stages of the union-forming process. We then use the estimated parameters to simulate each woman’s sequence of union transitions from ages 18–46, and use the simulated outcomes to predict probabilities that women with given characteristics follow a variety of long-term paths. We find that a typical, 18 year-old woman with no prior unions has a 22 % chance of cohabiting or marrying within 4 years and maintaining the union for 12+ years; this predicted probability remains steady until the woman nears age 30, when it falls to 17 %. We also find that unions entered via cohabitation contribute significantly to the likelihood of experiencing a long-term union, and that this contribution grows with age and (with age held constant) as women move from first to second unions. This finding reflects the fact that the high probability of entering a cohabiting union more than offsets the relatively low probability of maintaining it for the long-term. Third, the likelihood of forming a union and maintaining it for the long-term is highly sensitive to race, but is largely invariant to factors that can be manipulated by public policy such as divorce laws, welfare benefits, and income tax laws.