This paper documents increasing cohabitation in the United States, and the implications of this trend for the family lives of children. The stability of marriage-like relationships (including marriage and cohabitation) has decreased despite a constant divorce rate. Children increasingly live in cohabiting families either as a result of being born to cohabiting parents or of their mother s entry into a cohabiting union. The proportion of births to unmarried women born into cohabiting families increased from 29 to 39 per cent in the period 1980-84 to 1990-94, accounting for almost all of the increase in unmarried childbearing. As a consequence, about two-fifths of all children spend some time in a cohabiting family, and the greater instability of families begun by cohabitation means that children are also more likely to experience family disruption. Estimates from multi-state life tables indicate the extent to which the family lives of children are spent increasingly in cohabiting families and decreasingly in married families.