Book excerpt: Part of the Biology of Growing Animals series, this book presents up-to-date information on the biology of animal nutrition. It describes how dietary modulation of the gastrointestinal function in young and growing farm animals is achieved through different kinds of feed additives, such as probiotics, prebiotics, organic acids, and novel sources of feed enzymes, as well as bioactive components and metabolic modifiers. The book also discusses the role of nutrition in immune response and animal health, the problem of antinutrients - including mucotoxins and some minerals - in animal nutrition, and the biotechnological, molecular, and ecophysiological aspects of nutrition. In addition, safety and legal aspects are presented. Critical review and state-of-the art articles written by recognized specialsists in animal nutrition and gastrointestinal physiology Novel approaches for improving gastrointestinal function in young farm animals New ways of interpretation of basic knowledge of nutrition