Aging of the immune system, or "immunosenescence," is associated with both a marked reduction in responsiveness as well as functional dysregulation. These changes have been implicated in the increased morbidity and mortality of the elderly population from infectious disease, and may also play a role in autoimmunity and cancer. Though marginal alterations in B lymphocytes are apparent, the dramatic decline in humoral and cell-mediated responses is predominantly the consequence of alterations in the T-cell compartment. The effect of aging on CD4 cell function has been extensively summarized elsewhere. This review, therefore, focuses on the CD8 T-cell subset. Age-related changes in thymic function and involution, cellular homeostasis and lifespan, population shifts, T-cell activation, the process of replicative senescence, and oligoclonal expansions are discussed in terms of their effect on CD8 T cells. Age-associated alterations in CD4 T cells and antigen-presenting cells are mentioned insofar as these cells affect CD8 T-cell activation and function. Distinct patterns of immunosenescence in humans and mice are also noted.