Perceptual illusions refer to systematically oriented errors in the perception of figures or scenes, and these errors are observed in almost all people. For centuries, they have been called "opto-geometric illusions' because it was thought that they concerned only visual perception. Although visual errors are relatively rare, comments and questions about visual illusions are found as early as in the Greek and Roman literature. In scientific psychology, the theoretical and practical problems raised by these deformations have been intensively studied since the end of the 19th Century, and the elements of the figure inducing each error are now identified. However, there is no general theory explaining all the visual illusions. Instead, each figure must be analysed in order to determine the specific processes leading to the error.