Among several peculiarities of Lepidoptera cytogenetics, the WZ sex chromosome system is the most conspicuous. Whereas most animal and some dioecious plant species have an XY or X0 system, relatively few share the WZ or Z0 system with Lepidoptera, the most prominent ones being birds, snakes, and caddis ies (Trichoptera). Sex chromosomes and sex determination in Lepidoptera have been reviewed in a recent article (Traut, Sahara, and Marec 2007). We focus here on the fate of the W chromosome. Since most extant lepidopteran species possess a W chromosome, it may come as a surprise that the Lepidoptera clade started evolution with a Z0 system, and the W chromosome appeared much later in its history. Thereafter, as we will show, the evolution of the W chromosome followed the rules already learned from Y chromosomes: It decayed, fused with other chromosomes, and sometimes even got lost again. Hence, the W chromosome exhibits the full evolutionary life cycle of a typical sex chromosome in a single clade.