In 1909, Thayer referred to the visual effects created by many animal patterns as “illusions,” yet his ideas regarding the psychology of pattern perception and object recognition have not been explored further in this context. In contrast, there is considerable literature concerning the perceptual effects and mechanisms of visual illusions in humans. There is increasing evidence that non-human animals are susceptible to illusory effects and that some animals use illusions in behavioural contexts such as courtship. Using the foundation provided by research on human visual illusions, we discuss the role of illusions in animal signalling and what they can reveal about the cognitive mechanisms of image processing in non-human animals.