This longitudinal research examines maternal and child behaviors during joint planning over a 3-year period of middle childhood. 118 mother-child dyads were observed once a year beginning when the children were 8 years of age. Coding focused on mother and child planning behaviors, maternal instructional support, and child task engagement. Multilevel modeling was used to compare 3 models of stability and change: stability, normative developmental change, and individual differences in change. Results indicate that normative developmental change was the best predictor of mother and child planning behaviors and maternal guidance. Individual differences in rate of change predicted mothers' instructional support in the use of physical demonstration and child engagement measured by attention, task responsibility, and cooperation. Task difficulty contributed to these patterns. This research advances understanding of social interaction on cognitive tasks for partners in an established relationship. Implications for theory and future research are discussed.