During their intrathymic development, T lymphocytes acquire the ability to recognize myriads of antigens bound to self major histocompatibility molecules. This property is linked to the activation at early stages of thymic development of the biosynthesis of the antigen recognition receptors, the T cell receptors. It is also the consequence of intrathymic selective processes, that will favor maturation of thymocytes interacting with self histocompatibility products while preventing production of self-reactive lymphocytes. Here will be detailed the main intrathymic developmental stages and the mechanisms leading to the generation and intrathymic selection of the T cell repertoire.