Aging is associated with declines in multiple areas of immune function, but to date no single mechanism has emerged as being responsible for all the observed changes. Many changes occur at different rates within individuals as well as between individuals. With advancing age there is a concomitant increase in the incidence of many infections and cancers. It is being increasingly acknowledged that autoimmune processes play a proinflammatory role in the development of many pathological conditions, such as atherosclerosis. However, direct causal relationships between specific changes in immunity and the occurrence of specific diseases are rare. There is accumulating epidemiological, in vivo and in vitro evidence to support many such direct relationships in both animals and humans. It is likely that the mechanisms underlying age-related changes in immunity are multifactorial, with both genetic and environmental factors playing a significant role. Despite the current lack of unifying theories, much active and exciting work is proceeding in the area of immune stimulation. Studies describing age-related changes in immunity, as well as the testing of interventions to reverse these changes, will continue to fill the gaps in our knowledge, leading to a more comprehensive understanding of immunosenescence.