This article presents a framework for analyzing what it means to understand an intellectual skill. Building on the task analysis framework of Gagné and others, we suggest that many different types of knowledge may be associated with the core skills and subskills involved in a learning hierarchy. Second, we suggest that certain types of associated knowledge may lead to much greater understanding of the skill than will other types-an outcome we call skill plus productive knowledge. Then we present a classification system for analyzing the types of knowledge that are associated with skills concerning functional relations such as Ohm's Law. Finally, we suggest instructional prescriptions for developing learning outcomes that include appropriate associated knowledge. The article is intended as a framework for future research on understanding skills. The general direction of such research is outlined.