Selenium (Se) is an essential trace element of fundamental importance to health, primarily due to its antioxidant activity. Selenium has chemoprotective, anti-inflammatory, and antiviral properties and is related to enhancing immunity, controlling gastrointestinal, and reducing age-related disorders. In this book, the authors present current research in the study of the sources, functions and health effects of selenium. Topics discussed include selenium biorhythms and hormonal regulation; the physiological role of selenium; selenium supplementation of diet effects on antioxidant activity and protects against antibiotic resistance and avian influenza in poultry; selenium in critical illness and tropical diseases; products of animal origin as a source of Se in the human diet; selenium in food materials and stress signaling and proapoptotic pathways induced by selenite in malignant coloncytes.