Based on data from 710 2-parent families enrolled in the National Institute of Child Health and Human Development Study of Early Child Care and Youth Development, this article examined dyadic and family-level interdependence among indicators of family members' competence over time. A cross-lagged model that included children and both parents was used to simultaneously test relations among observed maternal sensitivity, observed paternal sensitivity, and children's externalizing behavior from 54 months to fifth grade. Testing 3 principal hypotheses, the study supported basic assumptions of a transactional family systems approach: (a) mother-child and father-child relations were independent predictors of change in children's and parents' behavior across middle childhood; (b) at all assessments, each parents' sensitive parenting predicted subsequent change in the other's sensitive parenting; and (c) both dyadic indirect effects between two family members and family-level indirect effects among all 3 family members were found. When predicting each members' behavior over time, a model that included both dyadic and family-level relations was superior to models that included only dyadic relations. Tests of 2 exploratory hypotheses suggested that (a) fathers' parenting predicted changes in mothers' parenting as equally as mothers' parenting predicted changes in fathers' parenting; and (b) mothers' parenting tended to be more influential early in development, and fathers' parenting was more influential later in development. The results suggest that individual development within families reflects complex dyadic and family-level interdependence among the behaviors of mothers, fathers, and their children over time.