Selenium (Se) is an essential dietary trace element participating in the regulation of various physiological functions in humans, farm animals and poultry through its incorporation into a range of selenoproteins. Low Se content in main feed ingredients is a common problem worldwide and dietary Se supplementation is a current practise in poultry and farm animal nutrition. Recent research clearly proved that sodium selenite, used for the last 40 years as a feed supplement, is not an optimal form of Se. However, use of organic selenium in animal/poultry diets can help meeting Se requirement and maintain high immunocompetence, productive and reproductive performance. The goal of this book is to provide up to date information about the roles of Se in poultry nutrition and health. A special emphasis is given to the role of selenium as an essential part of the integrated antioxidant system. Se in feed and organic Se are characterised in detail with emphasis to selenomethionine as a storage form of Se in the body. Also specific Se-deficiency related diseases in poultry are described and the importance of Se in growth, development, immunity and reproduction is demonstrated. A link between poultry industry and human health via production of Se-eggs and Se-enriched meat is shown. This book will be of practical importance to poultry producers, to nutritionists and vets as well as for avian/animal scientists, students of agricultural colleges and universities. It will also be of interest for researchers in areas related to ecology, environmental sciences, evolutionary biology, etc.