This paper reviews the roles of divorce, nonmarital childbearing, and cohabitation in the changing family contexts of children, and then provides new estimates of current family composition which incorporate cohabitation. The underlying process is viewed in terms of the declining significance of marriage linked to long-term trends in individuation. Half of all children in the U.S. will spend some time in a single-parent family, and nonmarital childbearing is an important factor creating these families. At the same time, increased cohabitation requires that family definitions based on marital status be replaced with those that include cohabitation. A sixth of traditionally defined "mother-only" families are cohabiting two-parent families, and the one-fourth of current stepfamilies that are cohabiting are missed by marriage-based definitions.