The performance of a decision bound model of categorization (Ashby, 1992a; Ashby & Maddox, in press) is compared with the performance of two exemplar models. The first is the generalized context model (e.g., Nosofsky, 1986Nosofsky, , 1992 and the second is a recently proposed deterministic exemplar model (Ashby & Maddox, in press), which contains the generalized context model as a special case. When the exemplars from each category were normally distributed and the optimal decision bound was linear, the deterministic exemplar model and the decision bound model provided roughly equivalent accounts of the data. When the optimal decision bound was nonlinear, the decision bound model provided a more accurate account of the data than did either exemplar model. When applied to categorization data collected by Nosofsky (1986Nosofsky ( ,1989, in which the category exemplars are not normally distributed, the decision bound model provided excellent accounts of the data, in many cases significantly outperforming the exemplar models. The decision bound model was found to be especially successful when (1) single subject analyses were performed, (2) each subject was given relatively extensive training, and (3) the subject's performance was characterized by complex suboptimalities. These results support the hypothesis that the decision bound is of fundamental importance in predicting asymptotic categorization performance and that the decision bound models provide a viable alternative to the currently popular exemplar models of categorization.Decision bound models of categorization (Ashby, 1992a; Ashby & Maddox, in press) assume that the subject learns to assign responses to different regions of perceptual space. When categorizing an object, the subject determines in which region the percept has fallen and then emits the associated response. The decision bound is the partition between competing response regions. In contrast, exemplar models assume that the subject computes the sum of the perceived similarities between the object to be categorized and every exemplar of each relevant category (Medin & Schaffer, 1978;Nosofsky, 1986). Categorization judgments are assumed to depend on the relative magnitude of these various sums.This article compares the ability of decision bound and exemplar models to account for categorization response probabilities in seven different experiments. The aim is