The vast majority of animals have intimate associations with bacteria that live inside them, sometimes as parasites, but often, as with gut flora, essential to their very existence. Recently, attention have been drawn to a class of bacteria that have profound implications for the evolution and ecology of the hosts they infect - bacteria that can manipulate their host's reproduction. So far, research has focused on invertebrates. This is the first comprehensive and systematic review of the activities of these reproductive parasites. It explores not only the intriguing theoretical questions raised by the relationships between symbiotic bacteria and their hosts, but also discusses a number of exciting practical applications, such as the potential for controlling disease-transmitting insects. Chapters on sex reversal, sex-ratio distortion, and evolution are included. This is a timely and up-to-the-minute exploration of a new, exciting, and rapidly expanding area of biology.