The thymus plays a key role in the development of the immune system. The function of the thymus is to establish and maintain the cellular immunity. During intrauterine development the environmental factors can represent the cause of various developmental anomalies of the thymus. Postnatally, these factors may cause primary immunodeficiency or diseases with deficient thymic functions. In newborns, the thymus is an organ with definitive morphology and function. In comparison with body proportions, the thymus reaches its biggest size in first six postnatal months. Then its lymphatic tissue is reduced and its function slowly decreases until the time of puberty. In spite of this physiological involution, this organ produces T-lymphocytes until the senescence. This article brings a review of normal thymic development, origin of developmental anomalies and sums up the knowledge of the structure, function and postnatal involution of the thymus.

        Key words: thymus development,microenvironment, thymus function, physiological involution of the thymus