T cell development starts with the entry of early thymic progenitor cells (ETP) through the cortico-medullary junction (CMJ) followed by their migration towards the subcapsular zone. IL-7Rα expression is initiated during this migratory phase, specifically in DN2 stage cells, which is critical for IL-7 dependent survival and proliferation of TCRβ-selected thymocytes. Upon further differentiation into DN4 and DP cells, IL-7Rα expression is terminated. DP thymocytes then reside in an IL-7 poor environment refractory to IL-7 signaling and pre-programmed to cell death. It is only after TCR-mediated positive selection that IL-7Rα expression is re-induced and that these cells become IL-7 signaling competent. Persistent TCR signaling desensitizes IL-7 signaling and permits CD4 lineage differentiation. Cessation of TCR signaling, however, allows IL-7R signaling in intermediate cells, and it is intrathymic IL-7 signaling that imposes CD8 lineage specification during CD4/CD8 lineage choice in the thymus.