Data from the 1997 National Survey of America's Families are being analyzed for a study of the effects of family structure on child well-being. The research is investigating the impact of factors such as parental psychological well-being, parent-child relationship quality, and family economic status. This chapter presents preliminary findings from the analysis in the context of the experience of children in cohabiting family units. The sample included 795 children living with two biological cohabiting parents and 1,049 children living with one biological parent and a partner. The children in cohabiting families are compared to children living with two married biological parents, married stepfamilies, single mother only, single father only, or no biological parents. The results indicate that parents in cohabiting stepfamilies experience more stress than biological parents who are married or living with the other biological parent of their children. The level of distress in cohabiting stepfamilies is equivalent to the stress of single parents. Children living in cohabiting stepfamilies also are less likely to be engaged in school than children living with two married or cohabiting biological parents. However, children living with unmarried biological parents have more emotional and behavioral problems than children whose parents are married. Children living with cohabiting parents also are less likely to go on outings than children living with married parents or stepfamilies. These preliminary findings indicate that cohabiting has an impact on child well-being and parental distress. Future studies should identify differences in the type of cohabitation relationships and the effect of child characteristics, including age, on the link between family structure and well-being. 17 references, 10 figures.