This paper briefly reviews research on tasks in knowledge-rich domains including developmental studies, work in artificial intelligence, studies of expert/novice problem solving, and information processing analysis of aptitude test tasks that have provided increased understanding of the nature of expertise. Particularly evident is the finding that expertise is acquired when people continually try to confront new situations in terms of what they know. Increasing ability to solve problems and generate new information is fostered by available knowledge that can be modified and restructured. Initial knowledge structures, when they are interrogated, instantiated, or falsified by novices in the course of learning and experience lead to organizations of knowledge that are the basis for the more complete schemata of experts. Acquiring expertise is seen as the successive development of procedurally oriented knowledge structures that facilitate the processes of expertise. A set of propositions is listed that summarizes conclusions from research as well as broader inferences and speculations. (PN)