Traditional theories of categorization in which categories are assumed to be grounded in perceptual similarity or theories ignore an important basis of conceptual structure: the emotion that a stimulus elicits in a perceiver. This article discusses the nature of, constraints on, and conditions of use of emotional response categories. Experiments in which participants sorted triads of concepts that shared both emotional and nonemotional relations indicate that individuals use emotional response categories when they are experiencing emotional states. Multidimensional scaling of similarity judgments by emotional and nonemotional perceivers supports a selective attention mechanism of these effects. Participants induced to feel happy or sad emotional states weighted the emotional responses associated with stimuli more heavily than people in relatively neutral states. The triad and multidimensional scaling findings, along with functional considerations, suggest that emotional response categorization is not only tenable, but necessary for a complete account of categorization.