The Müller-Lyer illusion is one of the best-known and most frequently examined optical illusions. After pointing out that it is unlikely that any one account would give a full explanation for all the features of this illusion, I argue for two claims. First, I aim to point out that an essential component of the Müller-Lyer illusion has something to do with picture perception (just as Gregory initially claimed). Second, I give an account of this essential component of the Müller-Lyer illusion that is not susceptible to the counterexamples and objections Gregorys inappropriate size constancy scaling theory was susceptible to. The gist of my account is that the Müller-Lyer illusion is explained not by inappropriate size constancy scaling, but by inappropriate shape constancy scaling.