Using data from Monitoring the FutureStudy that include 31 cohorts of high school seniors (1976 to 2006) who were followed from ages 19-30, weidentified heterogeneity in union formation trajectories andits covariates (cohort, sex, race/ethnicity, and parental education). We identified nine trajectorieswith approximately 40% following a single to married sequence (with variation in the timing of the sequence), about 35% remaining single, and the remaining respondents showing considerable heterogeneity.Recent cohortswere more likely to remain single and experience more transitions, women made earlier transitions, and Blackrespondents were less likely to follow pathways entailing marriage.