In recent years the gastrointestinal microflora has featured strongly in scientific, veterinary and medical research. As a result it has become obvious that the gut microflora is an essential component of the healthy animal. Not only is it involved in digestion of food, it is essential for the optimal resistance to disease. The first part of this book records the research that has been done on the factors affecting colonization of the gut and the effect that the flora has on the host animal. The second part discusses the way in which this basic knowledge affects the choice of organism being used as a probiotic. The evidence for the involvement of the gut microflora in the health and well-being of the animal is incontrovertible, but the development of probiotics has been largely empirical, failing to capitalize on the relevant research data. The bringing together of the basic information on gut microecology and the development of probiotic preparations is long overdue. It is hoped that this exercise will result in a more scientific approach to probiotic development and the emergence of new and improved preparations for animals and man. The authors involved are all experts in their field and I am greatly indebted to them for their contributions to the book. R. Fuller Abbreviations used for * generIc names Aspergillus A. B. Bacillus Bact. Bacteroides Bifidobacterium Bif. C. Clostridium Cam. Campylobacter Can. Candida Cor. Corynebacteri urn E. Escherichia Enterobacter Eb. Ent. Enterococcus Fusobacterium F. Fib. Fibrobacter K. Klebsiella 1.