In the name of Allah most gracious, most merciful. All praise and glory to Allah the almighty who made this small work to be accomplished and I am asking him to accept my efforts. Although toxic in large doses, selenium is an essential trace mineral in the animal diet and in some plants. It has a role in making antioxidant enzymes and a particular role in the functioning of the thyroid gland. This book contains three chapters about selenium in health and diseases. It provides readers with clear and simple information of the extraordinary story of selenium and its role in human health. It is written in a legible and user-friendly manner, and takes into account the considerable amount of fresh information that has been published over the past decade.