This article is a comprehensive overview of the biology of aging. The largely unproductive history of research on aging is presented, to lay the groundwork for a discussion of why such research is performed and what aging levels and human models are employed. The anatomic, biochemical, physiologic and behavioral measures used in such studies are delineated. Some of the most promising theories of aging (genetic, autoimmune, viral, free radical) are discussed, as are factors that could prolong the lifespan if optimally employed. Philosophic considerations are emphasized.