Objectives: This study examines the effects of young adult transitions into marriage and cohabitation on criminal offending and substance use, and whether those effects changed since the 1970s, as marriage rates declined and cohabitation rates rose dramatically. It also examines whether any beneficial effects of cohabitation depend on marriage intentions.Methods: Using multi-cohort national panel data from the Monitoring the Future (N = 15,875) study, the authors estimated fixed effects models relating within-person changes in marriage and cohabitation to changes in criminal offending and substance use.Results: Marriage predicts lower levels of criminal offending and substance use, but the effects of cohabitation are limited to substance use outcomes and to engaged cohabiters. There are no cohort differences in the associations of marriage and cohabitation with criminal offending, and no consistent cohort differences in their associations with substance use. There is little evidence of differences in effects by gender or parenthood.Conclusions: Young adults are increasingly likely to enter romantic partnership statuses that do not appear as effective in reducing antisocial behavior. Although cohabitation itself does not reduce antisocial behavior, engagement might. Future research should examine the mechanisms behind these effects, and why nonmarital partnerships reduce substance use and not crime.