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Kit ligand (KITL) is a pleiotropic growth factor that exerts an influence on target cells through binding the tyrosine kinase receptor, KIT. KITL and KIT are expressed by a variety of developmentally distinct cell lineages during both embryogen-esis and adult life, and roles for KITL and KIT in gametogenesis, melanogenesis, and hematopoiesis have been described. In mice KITL and KIT are products of the steel and white spotting loci, respectively, and mutations at these loci are associated with reduced fertility or sterility, depending on the nature of the defect. Numerous studies have shown that in the ovaries of rodents KITL and KIT are important for the migration, proliferation, and survival of primordial germ cells; primordial follicle activation; oocyte growth and survival; granulosa cell proliferation; the maintenance of meiotic arrest; theca cell recruitment; and the regulation of ovarian steroidogenesis. Therefore, KITL and KIT have diverse roles during oogenesis and folliculogenesis, and they are fundamentally important to fertility in rodents. However, many of these effects are yet to be confirmed in other species, including the human.