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Wolbachia pipientis are gram-negative bacteria that form intracellular inherited infections in many invertebrate hosts. They are extremely common with at least 20% of all insects being infected. Since insect species comprise ~85% of all animal species on the planet, Wolbachia pipientis are one of the most common bacterial endosymbionts in the biosphere and can be of major importance in ecological and evolutionary processes. Moreover they infect numerous non-insect invertebrates including filarial nematodes, terrestrial crustaceans, mites, and spiders. They are predominantly transmitted through females to developing eggs, but can also undergo some horizontal transmission between host species. The limits of the host range are not fully appreciated at this time. Much of the success of Wolbachia can be attributed to the diverse phenotypes that result from infection. These include classical mutualism in nematodes in which the bacteria are required for fertility and larval development; and reproductive parasitism in arthropods as characterized by the ability of Wolbachia to override chromosomal sex determination, induce parthenogenesis, selectively kill males, influence sperm competition and generate cytoplasmic incompatibility. Reproductive parasitism enhances the spread of Wolbachia through host arthropod populations by increasing the number of infected females, the transmitting sex of this bacterium. Wolbachia are present in mature eggs, but not mature sperm. It is thought that the phenotypes caused by Wolbachia, especially cytoplasmic incompatibility, may be important in promoting rapid speciation events in insects. The unique biology of Wolbachia has attracted a growing number of researchers and science educators interested in questions ranging from the evolutionary implications of infection to the use of this endosymbiont for human disease control and discovery-based projects in high school classrooms.