Using a longitudinal sample of 4,010 mothers and fathers from the Fragile Families and Child Wellbeing Study, we examine factors that predict whether children are living with both parents, only their mother, or only their father when the child is 3 years old. We considered parental characteristics and resources and couple\family-level characteristics and found that although many factors increased the odds of parents living together—including the financial resources of each parent, having a supportive relationship, and having a disability-free child—even more factors disproportionately increased the likelihood of either single fatherhood or single motherhood, including parents’ multiple-partner fertility and depression, mother’s drug use, and mother’s greater alcohol use. Our findings suggest that although most children living with a single parent live with their mothers, they are more likely to do so if their fathers exhibit problem behaviors, and more likely to live with their father if their mothers do so.