This scientific article offers a comprehensive overview of Wolbachia and its role in speciation. Sections include: What Wolbachia is; What Wolbachia Does to Its Host; Wolbachia – a Reproductive Parasite of Arthropods; Models for CI (Cytoplasmic Incompatibility); Theory of Coevolution of Wolbachia and Host Insect; Likely (Theoretical) Role of Wolbachia in Speciation; Drosophila as a Case Study; and What Defines a Strain of Wolbachia. A subscription to the Encyclopedia of Life Sciences may be required to view this article.