We use the National Longitudinal Study of Adolescent Health (Add Health) to examine whether childhood family structure experiences influence the development of romantic relationships during adolescence and whether adolescent relationships, in turn, help to shape long-term relationship trajectories. Young people who live in "nontraditional" families during their childhood are more likely than their peers to engage in romantic relationships during adolescence. Family-related mechanisms are significant mediators of this association. Individuals who were raised in stepparent and single-parent families are also more likely to cohabit during adulthood, and those who were raised in single-parent families are less likely to have ever married. Childhood family structure is not associated with serious relationship conflict during adolescence or adulthood, however. Moreover, although adolescent relationship experiences have long-term effects on relationship trajectories, they do not significantly mediate the associations between childhood family structure and relationship outcomes in adulthood.