Guided by the spillover hypothesis of family systems theory, we used data from the Building Strong Families project to examine intrafamily relations between destructive and constructive interparental conflict, harsh and supportive parenting behaviors, and coparenting alliance across different family configurations: married, cohabiting, and noncohabiting (never married) parents. Our sample (N = 2,784 couples/parents) was racially diverse, low-income couples/parents who were unmarried at the conception of their child. All variables were measured when children were approximately 36 months of age. Interparental conflict was assessed through an instrument that included both parents' reported conflict in the relationship and the perception of the other parent's conflict behaviors. Parenting behaviors were measured through observational data and coparenting alliance was based on mothers' and fathers' reports. As expected, path analyses revealed that destructive interparental conflict was related to lower levels of coparenting alliance, whereas constructive interparental conflict was related to higher levels of coparenting alliance, for mothers and fathers. For fathers only, destructive interparental conflict related to harsh parenting, suggesting that paternal parenting is vulnerable to the quality of the relationship with the mother. None of the proposed associations differed by family structure. These findings suggest the need for intervention programs to focus on promoting adaptive conflict management behaviors rather than on family structure. (PsycInfo Database Record (c) 2020 APA, all rights reserved).