For the last 25 years, Douglas Medin has been one of the leading researchers in the domain of categories. He has also been quite influential in the field, and much of the first author's thinking about categories began when the first author was a new faculty member at the University of Illinois with an office next to Professor Medin's (with a brief switch to the first person for ease of reading). Medin was a great mentor (and still is), and he offered to read everything I wrote and comment on it. It was wonderful to get expert advice on my writing and thinking. In my exuberance (I was young), I offered to reciprocate, to read and comment on everything he wrote. Well, it did not take me long to discover that he was writing about 10 times as much as I was. However, given the quality of the comments, I think I still came out ahead. In addition, I had a chance to read and discuss his work with him (plus, I realize now that he was not giving me everything he was writing). I still find myself sometimes including points or ways of expressing something that I can trace back to the comments Medin gave me. Most important, he questioned everything. I would often give him background on a problem I was thinking about, explain my ideas, design, and predictions. He would listen politely and then ask whether a basic assumption of mine (and the field's) might be wrong. He might say, "Yes, that sounds good, or maybe it is that . . ." and then question the point I had begun with. It would drive me crazy, but eventually I think I understood his "suggestion" about how to approach a research topic. This chapter is one attempt to see if I did.Categories are critical for a wide variety of cognitive activities, such as classification, inference, problem solving, explanation, and communication. Thus an understanding of categories is a critical part of understanding cognition. The goal of this chapter is to examine a central aspect of the study of category learning, classification learning, and reconsider its role.We thank the editors of this volume for their wonderful organization of the festschrift. We were helped greatly by the comments of Art Markman and Gregory Murphy. Last, but not least, we thank Douglas Medin for his influence and friendship.