We examine the life course transitions into and from families headed by unmarried cohabiting couples for a recent cohort of American children. Life table estimates, based on data from the National Longitudinal Survey of Youth mother-child files, indicate about one in four children will live in a family headed by a cohabiting couple sometime during childhood. Economic uncertainty is an important factor determining whether children in single-parent families subsequently share a residence with a mother's unmarried partner. Moreover, virtually all children in cohabiting-couple families will experience rapid subsequent changes in family status. Our estimates provide a point of departure for future work on children's exposure to parental cohabitation and its social and economic implications.