One of the most important aspects of the perceptual process is its speed. The percept must be rapid and judgments made on the basis of the stimulus input must be immediate, so that the organism can adapt as rapidly as possible to a change in the environment. When instances of perceptual error based on global impressions are considered, the analogy to visual illusions is an obvious one. This chapter describes an illusion as a test element and a series of accessory lines. Since no illusion exists when the test element is presented alone, it seems reasonable to assume that the accessory lines are responsible for the obtained distortion. One of the best configurations with which to demonstrate the operation of confusion and averaging theories is the Mueller-Lyer. The concept of different perceptual styles, incorporating differential tendencies to use averaging strategies, may also prove useful in understanding some of the observed changes in illusion magnitude that occur with age.