Families formed by unmarried parents increased dramatically in the United States during the latter half of the 20th century. To learn more about these families, a team of researchers at Princeton University and Columbia University designed and implemented a large birth cohort study—The Fragile Families and Child Wellbeing Study. This chapter highlights several findings from the study. First, most unmarried parents have “high hopes” for a future together at the time of their child’s birth; but their resources are low and most relationships do not last. Second, unmarried mothers experience high levels of partnership instability and family complexity, both of which are associated with lower-quality parenting and poorer child well-being. Finally, welfare state, child support and criminal justice policies play a large role in the lives of fragile families.