The parent-child relationship is critically important for children's functioning and long-term outcomes. Although typically measured by self-report or global codes in observed interactions, parent-child interactions actually occur on a moment-to-moment basis, with frequent shifts in behavior and affect happening in each member of the dyad. Even so, moment-to-moment interactions in these dyads are rarely studied. We sought to illuminate how complementarity, or the extent to which behavior in one member of the dyad shapes that of the other, impacts the quality of the parent-child relationship. Parent-child dyads in 1,030 families completed a cooperative video-recorded task, after which each member of the dyad was rated on warmth and control twice a second. Results illustrated high levels of warmth and control complementarity in parent-child relationships, with mothers showing more complementarity than fathers and greater control complementarity relative to warmth complementarity. Results showed mother-child and father-child warmth complementarity was associated with increased parent-child reciprocity, whereas mother-child and father-child control complementarity was associated with increased parent-child cooperation. In addition, father-child warmth complementarity was associated with a decrease in observed father-child conflict and an increase in father-child cooperation. Finally, father-child control complementarity was associated with a decrease in observed father-child reciprocity. However, no significant associations were found between complementarity and family reports of parental involvement or conflict with child. Results highlight complementarity as an important part of parent-child interactions but also indicate it is relationship- and domain specific. (PsycInfo Database Record (c) 2022 APA, all rights reserved).