Developmental and individual differences in planning were assessed by administering a computerized planning task to three groups of children. Computer protocols provided "product data" (performance time, accuracy, number of moves, etc.) and participants' think-aloud protocols were collected to obtain "process data." Results suggested that due to high within-group variability, product variables frequently failed to show developmental differences. Process data identified several sources of this variability, such as differences in plan-formation and execution, as well as considerable intraindividual variability across items. The importance of process data to the assessment of planning it also discussed.