Two prominent immunogerontologists review the major features and functions of the immune system that are most likely, or known, to be significantly altered by aging, and offer insightful analyses of the consequences for those aging subjects who must cope with infection. Topics of special interest include the demographics and theories of immunosenescence, the gradual breakdown of resistance to infection in the aged, and the effects of aging on selected mechanisms of both innate and adaptive immunity to infections. The Albrights also explain how advances may be made in understanding the basic biology, the newer methods of treatment and prevention, and the evaluation of such provocative ideas as lifespan extension and nutritional intervention to delay immunosenescence.