This paper discusses research on the processes by which individuals progress toward expertise in scientific domains, focusing on the activity of constructing qualitative problem representations during solution of standard mechanics problems in physics. It includes information on: (1) descriptive analyses of problem-solving performance; (2) prescriptive models of problem-solving performance (which specify the knowledge required for novices to perform well on particular tasks); (3) a prescriptive theory of problem description (which specifies a procedure leading to construction of correct mechanics problem descriptions); (4) a theory of instruction which incorporates a particular model of learning-centered coaching referred to as "Evolving External Control Knowledge" (EECK); (5) the development of computer-oriented instructional materials based on the prescriptive theory and the EECK; and (6) preliminary results, focusing on possible relationships between changes in subjects' (N=9) performance and features of both the model of performance and the model of instruction. Results (among others) indicate that subjects seemed to be internalizing the notion of force as a kind of interaction and that the instructional materials are differentially appropriate for individuals at different stages of learning. (JN)