How have recent changes in U.S. family structure affected the cognitive, social, and emotional well-being of the nation's children? Paul Amato examines the effects of family formation on children and evaluates whether current marriage-promotion programs are likely to meet children's needs. Amato begins by investigating how children in households with both biological parents differ from children in households with only one biological parent. He shows that children growing up with two continuously married parents are less likely to experience a wide range of cognitive, emotional, and social problems, not only during childhood but also in adulthood. Although it is not possible to demonstrate that family structure causes these differences, studies using a variety of sophisticated statistical methods suggest that this is the case. Amato then asks what accounts for the differences between these two groups of children. He shows that compared with other children, those who grow up in stable, two-parent families have a higher standard of living, receive more effective parenting, experience more cooperative co-parenting, are emotionally closer to both parents, and are subjected to fewer stressful events and circumstances. Finally, Amato assesses how current marriage-promotion policies will affect the well-being of children. He finds that interventions that increase the share of children who grow up with both parents would improve the overall well-being of U.S. children only modestly, because children's social or emotional problems have many causes, of which family structure is but one. But interventions that lower only modestly the overall "share" of U.S. children experiencing various problems could nevertheless lower substantially the "number" of children experiencing them. Even a small decline in percentages, when multiplied by the many children in the population, is a substantial social benefit. (Contains 2 tables and 68 endnotes.)