Until the mid-20th century, infectious diseases were the major cause of morbidity and mortality in humans. Massive vaccination campaigns, antibiotics, antivirals, and advanced public health measures drastically reduced sickness and death from infections in children and younger adults. However, older adults (> 65 y of age) remain vulnerable to infections, and infectious diseases remain among the top 5–10 causes of death in this population. The aging of the immune system, often referred to as immune senescence, is the key phenomenon underlying this vulnerability. This review centers on age-related changes in T cells, which are dramatically and reproducibly altered with aging. I discuss changes in T cell production, maintenance, function, and response to latent persistent infection, particularly against CMV, which exerts a profound influence on the aging T cell pool, concluding with a brief list of measures to improve immune function in older adults.