Wolbachia pipientis is an intracellular bacteria found within the cytoplasm of a high proportion of arthropods. Widespread in insects, Wolbachia is also commonly found in other arthropod groups, including mites, spiders and terrestrial isopods. Wolbachia are normally maternally inherited and have evolved a number of strategies to ensure transmission. These include: (1) feminization, the conversion of genetic males into females, (2) parthenogenesis, the production of diploid offspring without sexual reproduction, (3) male killing, the killing of infected males to the benefit of infected female siblings, and (4) cytoplasmic incompatibility, the inability of infected males to successfully fertilize eggs from either uninfected females or females infected with different Wolbachia types. In addition to this reproductive parasitism, Wolbachia can influence other aspects of host fitness, including host longevity, fecundity, fertility and host-parasitoid interactions. Wolbachia has been studied extensively in the context of host spermatogenesis, oogenesis and embryogenesis. These cytological studies of host-Wolbachia interactions are providing insights into the ways in which Wolbachia are able to manipulate hosts.