In humans, CD56, CD161, CD16, CD94 or CD57 represent prototypic markers of NK cells, although they are also found on a subset of CD8+ T cells. We propose here that the expression of NK receptors on CD8+ T cells can be considered a marker of cytotoxic effector T cells that are expanded in vivo after antigenic activation leading to extensive proliferation. The persistence of antigen will lead to loss of co-stimulatory molecules, telomere shortening and defective IL-2 production, changes that define the state replicative senescence in T lymphocytes. The majority of these "effector/senescent" T lymphocytes are CD8+, CD45RA+, CD11a(bright), CD28-, CD27-, CD62L- and CCR7-. They are cytotoxic T cells with strong expression of intracytoplasmic perforin and granzyme, but with low proliferative capacity and defective IL-2 production. Many of these characteristics are shared by the recently defined "effector/memory" T cells, being mainly distinguished by the absence of CD45RA expression on the memory cells. The expression of NK receptor in these effector cells will probably contribute to the regulation of their cytotoxic function. Expansion of cells with these characteristics can be found not only in the elderly but also in other clinical conditions involving chronic activation of the immune system such as viral infections, rheumatic and autoimmune diseases or tumors. Another subset of T cells that expresses the NK receptors is the alpha-galactosyl-ceramide specific T cell subset defined by the expression of canonical Valpha24JalphaQ TCR, recognition of CD1d and secretion of high amounts of IL-4 and IFN-gamma. However, the changes observed in the expression of NK-R on T cells associated to immunosenescence can not be attributed to expansion of this particular T cell subset, although alterations in the number and function of these cells have been demonstrated in some autoimmune diseases.