As nonmarital childbearing escalated in the United States over the past half century, fragile families--defined as unmarried couples with children--drew increased interest from researchers and policy makers. Sara McLanahan and Audrey Beck discuss four aspects of parental relationships in these families: the quality of parents' intimate relationship, the stability of that relationship, the quality of the co-parenting relationship among parents who live apart, and nonresident fathers' involvement with their child. At the time of their child's birth, half of the parents in fragile families are living together and another third are living apart but romantically involved. Despite high hopes at birth, five years later only a third of parents are still together, and new partners and new children are common, leading to high levels of instability and complexity in these families. Drawing on findings from the Fragile Families and Child Wellbeing Study, McLanahan and Beck highlight a number of predictors of low relationship quality and stability in these families, including low economic resources, government policies that discourage marriage, gender distrust and acceptance of single motherhood, sex ratios that favor men, children from previous unions, and psychological factors that make it difficult for parents to maintain healthy relationships. No single factor appears to have a dominant effect. The authors next discuss two types of experiments that attempt to establish causal effects on parental relationships: those aimed at altering economic resources and those aimed at improving relationships. What can be done to strengthen parental relationships in fragile families? The authors note that although economic resources are a consistent predictor of stable relationships, researchers and policy makers lack good causal information on whether increasing fathers' employment and earnings will increase relationship quality and union stability. They also note that analysts need to know more about whether relationship quality in fragile families can be improved directly and whether doing so will increase union stability, father involvement, and co-parenting quality.