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The development and maintenance of color polymorphism in cryptic prey species is a source of enduring fascination, in part because it appears to result from selective processes operating across multiple levels of analysis, ranging from cognitive psychology to population ecology. Since the 1960s, prey species with diverse phenotypes have been viewed as the evolved reflection of the perceptual and cognitive characteristics of their predators. Because it is harder to search simultaneously for two or more cryptic prey types than to search for only one, visual predators should tend to focus on the most abundant forms and effectively overlook the others. The result should be frequency-dependent, apostatic selection, which will tend to stabilize the prey polymorphism. Validating this elegant hypothesis has been difficult, and many details have been established only relatively recently. This review clarifies the argument for a perceptual selective mechanism and examines the relevant experimental evidence.