The ability to compare two objects is fundamental to intelligent behavior. However, it is important to distinguish between the implicit comparison that plays a role in all categorization and explicit comparison, by which two object representations are compared in short-term memory. Children learn early on both to categorize and to perform explicit comparison, but it requires a long time for them to learn to categorize and compare along particular dimensions. In this paper we present a connectionist model which brings together categorization and comparison, focusing on the development of the use of dimensions in both processes. The model posits a very general comparison mechanism which is blind to the nature of its inputs and sharing of internal object and dimension representations by categorization and comparison processes. As the system is trained on the two processes, it learns to use dimension inputs as filters on the internal representations for input objects; it is these filtered representations which are matched in comparison. The model provides a natural account of the tendency for early comparison along a single dimension to be disrupted by similarities along other, irrelevant dimensions and of the process through which the child might overcome this deficiency.