This Article argues that U.S. law should give protection to relationships between cohabitants and their partners’ children when necessary to avoid the economic and emotional trauma that may be caused by separation of the child from a member of his or her household if the cohabitation ends. After examining the social science literature about the welfare of both stepchildren and children of cohabitants and the inadequate legal treatment of custody, visitation, and child support issues under current law, the author recommends that cohabiting stepparents (1) be given standing to seek custody if they have acted as de facto parents, with a presumption in favor of custody by the stepparent when the cohabitant who is the child’s biological parent has died and the possibility of joint custody in other cases; (2) be awarded visitation if both the ex-cohabitant and child desire it; and (3) be obligated to pay child support for the child if the cohabitation dissolves after a period of two years or more.