The Preface to the first edition of this book explained the reasons for the publication of a comprehensive text on the rumen and rumen microbes in 1988. The microbes of the ruminant's forestomach and those in related organs in other animals and birds provide the means by which herbivorous animals can digest and obtain nutriment from vegetation. In turn, humans have relied, and still do rely, on herbivores for much of their food, clothing and motive power. Herbivores also form the food of carnivorous animals and birds in the wild. The importance of the rumen microorganisms is thus apparent. But, while a knowledge of rumen organisms is not strictly neces sary for the normal, practical feeding of farm animals, in recent years there has been much more emphasis on increasing the productivity of domesti cated animals and in rearing farm animals on unusual feedstuffs. Here, a knowledge of the reactions of the rumen flora, and the limits to these reactions, can be invaluable. In addition, anaerobic rumen-type microor ganisms are found in the intestines of omnivores, including humans, and can be implicated in diseases of humans and animals. They are also found in soils and natural waters, where they playa part in causing pollution and also in reducing it, while the same organisms confined in artificial systems are essential for the purification of sewage and other polluting and toxic wastes.