Psychological studies of categorization often assume that all concepts are of the same general kind, and are operated on by the same kind of categorization process. The authors argue against this unitary view, and for the existence of qualitatively different categorization processes, particularly a distinction between categorizing an item by applying a category-defining rule to the item vs determining the similarity of that item to remembered exemplars of a category. After characterizing rule application and similarity computations as strategies of categorization, the authors review studies that have used artificial categories and shown that differences in instructions or time pressure can lead to either rule-based or similarity-based categorization. Also considered are studies that have used natural concepts and again demonstrated that categorization can be done by either rule application or similarity calculations. Evidence from cognitive neuroscience relevant to the rule vs similarity issue is offered, as well as the results of a recent neuroimaging experiment which indicates that different neural circuits are involved when people categorize items on the basis of a rule as compared with when they categorize the same items on the basis of similarity.