This study clarifies within-family and between-family links between marital functioning and child well-being. Expanding on existing prospective research, this study tests whether changes in parents' marital functioning are associated with corresponding changes in their children's well-being, independent from associations that exist when comparing different families. Participants (N= 1,033) were members of married, opposite-sex couples with children who participated in five waves of a larger study of marriage in the U.S. Army. Spouses' constructive communication, verbal conflict, and marital satisfaction each showed between-family associations with parent-reported child internalizing and externalizing problems. In contrast, within-family associations were significant only for parents' communication behaviors. That is, parents who reported lower levels of marital satisfaction also reported lower child well-being, whereas change in parents' communication was associated with change in child well-being over time. Isolating within-family effects is important for understanding marital and child functioning and for identifying potential targets for effective intervention.