The capacity of the immune system to mediate effective immune responses to pathogens declines with age. In the case of immune responses to newly encountered antigens, several studies have demonstrated that this decline reflects both a loss of naïve T cells and changes in the repertoire and function of these cells over time. However, comparatively little is known about the impact of age on established memory T cells pools. Here we discuss age-related changes in memory CD8 + T cell pools elicited by influenza and parainfluenza viruses and the impact of these changes on immunity in general.A large number of studies have documented that increasing age is associated with a progressive decline in the capacity to mount effective immune responses (Burns et al., 1993;Cook et al., 1987;Effros, 2003;Grubeck-Loebenstein and Wick, 2002;Linton and Dorshkind, 2004;McElhaney, 2003;McElhaney, 2005;Miller, 1991;Miller, 1996;Murasko and Jiang, 2005;Nagelkerken et al., 1991;Phair et al., 1978;Wick et al., 2000;Zheng et al., 1997). Aged individuals typically have an impaired capacity to clear infections and also exhibit significant defects in graft rejection, delayed type hypersensitivity, and tumor rejection (Haynes and Eaton, 2005;Miller, 1996;Po et al., 2002). These defects have important clinical consequences for the elderly in terms of susceptibility to infection and poor vaccine efficacy. Recent studies have implicated both the cellular and humoral arms of the immune response in this age-related decline in immune function. For example, studies in animals and humans have shown that aged individuals have a significantly reduced capacity to mount an effective antibody response following infection or vaccination, which is thought to be a consequence of dysfunctional aged CD4 + T cell "help" (High, 2004;Johnson and Cambier, 2004). In addition, involution of the thymus results in a decline in naïve T cell numbers, potentially limiting the breadth and quality of the repertoire of the T cell response (Naylor et al., 2005). However, we still lack a solid understanding of the scope of immune defects or mechanisms underlying the decline in immune efficacy in the elderly.Whereas most studies addressing the impact of aging on immunity have focused on immune responses to newly encountered antigens, relatively little work has addressed the impact of age on pre-existing memory T cell populations. Studies by Kapasi et al using a systemic viral model in mice demonstrated that CD8 + T cell memory remains functional for over a year after its initial generation (Kapasi et al., 2002). This is consistent with human studies suggesting that cellular immunity is relatively long lived (Hammarlund et al., 2003). However, the relative efficacy of the recall of long-term T cell memory (ie. memory that was originally established