Many feel that an advantage to using computer simulations in science teaching is that they give students the opportunity to witness or perform experiments which might otherwise be too expensive, time consuming, or dangerous for them to do. Simulations attempt to mimic the kind of experience students get in a laboratory and help students learn new facts. It is held that simply providing students with laboratory experience does not help them to understand the ideas of scientists. This paper urges researchers, educators and software designers to consider how computer simulations can be used to bring about conceptual change and understanding. The purpose of this paper is to provide a rationale for designing conceptually enhanced simulations and describe their underlying structure. Part one discusses natural phenomena and defines the problem. Part two discusses model systems and identifies the kinds that may play a role in helping students understand the theoretical frameworks of scientists. The third part shows how these ideas were applied in developing a simulation to teach the concepts of weight and density. Twenty-five references are listed. (CW)