Two concepts capture the dynamic and complex nature of contemporary family structure: family instability and multipartner fertility. Although these circumstances are likely to co-occur, their respective literatures have proceeded largely independently. The authors used data from the Fragile Families and Child Wellbeing Study (N = 3,062) to consider these dimensions of dynamic family structure together, asking whether they independently predict children's behavior problems at age 9. Frequent family instability was consistently predictive of higher predicted levels of behavior problems for children born to mothers who were unmarried, an association largely attenuated by factors related to family stress. Multipartner fertility was robustly related to self-reported delinquency and teacher-reported behavior problems among children born to mothers who were married.