An age-related decline in immune functions, referred to as immunosenescence, is partially responsible for the increased prevalence and severity of infectious diseases, and the low efficacy of vaccination in elderly persons. Immunosenescence is characterized by a decrease in cell-mediated immune function as well as by reduced humoral immune responses. Age-dependent defects in T- and B-cell function coexist with age-related changes within the innate immune system. In this review, we discuss the mechanisms and consequences of age-associated immune alterations as well as their implications for health in old age.