Wolbachia are a group of obligatory intracellular and maternally inherited bacteria found in many arthropod species, including insects, mites, spiders, springtails, crustaceans, as well as in certain nematodes. Several PCR-based surveys suggest that over 20% of the arthropod species may be Wolbachia-infected, rendering this bacterium the most ubiquitous intracellular symbiont yet described. Wolbachia have recently attracted attention for their potential as novel and environmentally friendly bio-control agents. Wolbachia are able to invade and maintain themselves in the arthropod species through manipulation of the host's reproduction. Several strategies can be distinguished, one of which is cytoplasmic incompatibility (CI). Wolbachia-induced cytoplasmic incompatibility can be used beneficially in the following ways: (a) as a tool for insect pest population control in a way analogous to the "Sterile Insect technique" (SIT) and (b) as a drive system to spread desirable genotypes in field arthropod populations. In addition, virulent Wolbachia strains offer the potential to control vector species by modifying their population age structure. In the present chapter, I summarize the recent developments in Wolbachia research with an emphasis on the applied biology of Wolbachia and conclude with the challenges that Wolbachia researchers will face if they want to use and/or introduce Wolbachia into pest and vector species ofeconomic, environmental and public health relevance and, through Wolbachia-based technologies, to suppress or modify natural populations.