Publisher Summary Polymorphism is the occurrence together in the same environment of two or more discontinuous forms of a species in such proportions as the rarest of them cannot be maintained merely by recurrent mutation. Two distinct types must be recognized: balanced polymorphism which is due to a balance of selective agencies that favor diversity and transient polymorphism, which exists only while a gene spreads and displaces its allelomorph. This chapter presents an account of polymorphism in the Lepidoptera: in species with a dimorphism affecting both sexes, in those with multiple forms, and where sex-controlled inheritance limits the polymorphism to males or females. Instances are described in which polymorphism affects the habit of the contrasted forms; therefore, some light has been thrown on their maintenance. The most striking example of transient polymorphism in the Lepidoptera and indeed in any organism, animal or plant, is provided by industrial melanism. Further, a general survey of the control of mimetic polymorphism is given. It is shown that Batesian mimics will usually tend to be polymorphic, and the different phases will be adjusted to such frequencies that they each possess equal advantages, whether all or only some of them are mimetic.