The germfree animal is reared in the laboratory to be bacteria free; its counterpart, the gnotobiotic animal, is exposed to select microorganisms. The need for such an animal model for use in biomedical studies was first expressed by Pasteur in the late 1800s. Subsequent development of germfree and gnotobiotic animals led to an explosion of studies on the effects of microflora and its components on the physiology and metabolism of the host. Germfree and Gnotobiotic Animal Models brings together the most notable points of early and recent studies and gives reference to the most pertinent literature.