R. M. Nosofsky and T. J. Palmeri's (1997) exemplar-based random-walk (EBRW) model of speeded classification is extended to account for speeded same-different judgments among integral-dimension stimuli. According to the model, an important component process of same-different judgments is that people store individual examples of experienced same and different pairs of objects in memory. These exemplar pairs are retrieved from memory on the basis of how similar they are to a currendy presented pair of objects. The retrieved pairs drive a random-walk process for making same-different decisions. The EBRW predicts correctly that same responses are faster for objects lying in isolated than in dense regions of similarity space. The model also predicts correctly effects of same-identity versus samecategory instructions and is sensitive to observers' past experiences with specific same and different pairs of objects.The main tenet of exemplar-based models of cognition is that people store particular instances of events in memory, called exemplars, and that these exemplars are later retrieved to perform a particular task. Exemplar-based models have long been used to model performance in categorization tasks (Medin & Schaffer, 1978). These models assume that people represent categories as a set of exemplars and make categorization decisions by retrieving exemplars from memory. Such models have rendered accurate quantitative accounts of category learning, transfer, and generalization (Hintzman, 1986; Krnschke, 1992;Nosofsky, 1986).Recently, Nosofsky and Palmed (1997) proposed an exemplarbased model of classification, which also accounts for the timecourse of classification. This exemplar-based random-walk (EBRW) model posits that people store category examples along with their category labels in memory. These exemplars are assumed to reside in a multidimensional psychological space. During a classification judgment, exemplars race to be retrieved. How fast an exemplar races is determined by its similarity to the test item. The winning exemplar adds incremental evidence to a random walk process. A response occurs when the random walk counter reaches a criterion. The EBRW correctly predicts effects of withinand between-category similarity, practice, and familiarity on classification response times and accuracies. This model is discussed in more detail below.Exemplar-basad models have also been successful in other domains. For example, Logan's (1988Logan's ( , 1992 instance theory of automaticity construes skilled action as a race between algorithmic and instance-based processes. He suggests that to perform a task