Aging is a complex process (or series of processes). Recent evidence suggests that several of its most important mechanisms are linked by means of cellular damage caused by reactive oxygen species (ROS). Oxidative damage may be a major factor in the loss of physiological functions that occur in degenerative diseases and aging. This is because, in aerobic organisms, the mitochondrial electron transport chain plays an important role in energy production and is a significant source of ROS that damage DNA, RNA, and proteins in cells. While oxidative events in other cell organelles are likely to contribute to the pathobiology of aging, this review highlights alterations in mitochondrial function that, due to accumulated oxidative damage, occur with age.