Although an extensive literature has shown that family structure is linked with child well-being, less well understood is how the dynamics within families affect children, in particular the extent to which positive mother—father relationship quality is linked with children's outcomes. In this study the authors used data from the Fragile Families and Child Wellbeing Study (N = 773) to examine how couple supportiveness in stable coresident families is related to children's externalizing and internalizing behavioral problems over ages 3 through 9. Using latent growth curve and fixed effects models, they found that parents' greater supportiveness has a slight association with lower levels of children's behavioral problems. Using cross-lagged structural equation models to examine the direction of the association, they also found some evidence that parents' relationship quality and children's behavioral problems are reciprocally related. Overall, this study suggests that more positive couple interactions are beneficial for children residing with both of their biological parents.