The epithelial cells of the thymus synthesize at least 30 different polypeptides: the thymic hormones. The structure of 4 of them is well known. They are named thymosin alpha 1, thymopoietin, thymulin and thymic humoral factor. Biological functions and secretion regulation of thymic hormones are described as well as the interactions between brain, thymus and endocrine glands. Blood levels and clinical usefulness of thymic hormones are reviewed in different congenital or acquired immunodeficient states and in autoimmune diseases.