Intrathymic T cell development represents one of the best studied paradigms of mammalian development. Lymphoid committed precursors enter the thymus and the Notch1 receptor plays an essential role in committing them to the T cell lineages. The pre-T cell receptor (TCR), as an autonomous cell signaling receptor, commits cells to the αβ lineage while its rival, the γδTCR, is involved in generating the γδ lineage of T cells. Positive and negative selection of immature αβTCR-expressing cells are essential mechanisms for generating mature T cells, committing them to the CD4 and CD8 lineages and avoiding autoimmunity. Additional lineages of αβT cells, such as the natural killer T cell lineage and the CD25+ regulatory T cell lineage, are formed when the αβTCR encounters specific ligands in suitable microenvironments. Thus, positive selection and receptor-instructed lineage commitment represent a hallmark of the thymus. Ectopically expressed organ-specific antigens contribute to thymic self–nonself discrimination, which represents an essential feature for the evolutionary fitness of mammalian species.