It is well established that increasing age is associated with a decreased capacity of the immune system to mediate effective immune responses to vaccination and invading pathogens. Because of the inherent limitations of conducting experiments in humans, much of what we have learned is owed to the utility of experimental mouse models of aging. Recent studies performed in the mouse have demonstrated mechanisms responsible for age-related declines in the function of CD4⁺ and CD8⁺ cells. This review describes key findings regarding age-related defects in T-cell function and discusses the impact these defects have on vaccine efficacy and immunity.