ABSTRACT: ATRIUM, a rule-and-exemplar theory of category learning, posits that people can learn to use different psychological representations to classify different stimuli. This entails that people's representations will change from stimulus to stimulus at a given time and will change over time. Experiment 1 extended work by Aha and Goldstone (1992) to demonstrate that participants utilize multiple representations once training is complete and that the representation that is selected varies systematically from stimulus to stimulus. Experiment 2 extended work by Nosofsky, Clark, and Shin (1989) to demonstrate that many participants use exemplar-similarity based representations during training and shift to rule representations when feedback is eliminated and participants are required to generalize to novel stimuli. ATRIUM is able to account for the data and able to assay participants' representations.