Positive selection occurs in the thymic cortex, but critical maturation events occur later in the medulla. We defined the precise stage at which T cells acquire competence to proliferate and emigrate. Transcriptome analysis of late gene changes suggested roles for NF-B and interferon signaling. Mice lacking the IKK kinase TAK1, showed normal positive selection, but a specific block in functional maturation. NF-B signaling provided protection from TNF, and was required for proliferation and emigration. Alternatively, the interferon signature was independent of NF-B, and IFNR deficient thymocytes showed reduced STAT1 levels and phenotypic abnormality, but were competent to proliferate. Thus, both NF-B and tonic IFN signals are involved in the final maturation of thymocytes into nave T cells.