This is an extensive review of Wolbachia biology published in 1997. Topics covered include history of discovery and study, phylogeny and distribution, Wolbachia-induced alterations in host reproduction, population biology and evolution, and biological control implications. This review is more than 10 years old, so some points relating to Wolbachia molecular biology and genetics are out of date. However, the basics are well covered. This would make good background reading for undergraduate or higher level students beginning to study Wolbachia.