Scientists and researchers in many disciplines frequently resort to modeling and model–based reasoning to concretize abstract ideas, to simplify and clarify complex phenomena, to predict trends, and to explain mechanisms and processes. National projects working to reform science education explicitly recommend the development of an appreciation for the centrality of models in the teaching and learning of science. This article describes a model–centered, computer–supported, semester–long science curriculum for middle–school students designed to encourage conceptual understanding and to foster the development of model–based reasoning skills. In the Model–based Analysis and Reasoning in Science (MARS) project, an attempt is made to create an environment conducive to fostering conceptual understanding and reasoning about scientific phenomena that involve “balance of forces.” Visual models that are dynamic and interactive are presented not only to concretize abstract ideas but also as reasoning tools that give students the leverage to solve problems in a variety of contexts. This model–centered curriculum focuses on a network of concepts important for understanding hydrostatics; that is, floating and sinking, as an exemplar for the general principle of balance of forces. How students understand these concepts and use models as a disciplinary resource to engage in chains of reasoning that integrate concepts into networks of relations is of special interest for study. How students use that understanding in new situations within the same explanatory system is also being studied. A major pedagogical question is how middle–school students can be taught to engage in model–based reasoning, a form of reasoning that scientists routinely use. © 1995 John Wiley & Sons. Inc.