An introductory open access review paper looks briefly at some of the theories of aging: "Ageing and senescence are related words and are often used interchangeably as both processes are characterized by progressive changes in the tissue of the body, eventually leading to a decline in function and death of the organism. Senescence refers to a post-maturational process that leads to diminished homeostasis and increased vulnerability of the organism to death. Ageing, in contrast, refers to any time-related process and is a continuous process that starts at conception and continues until death. The mechanisms involved in ageing are partially intrinsic to the organism, like genetic and epigenetic factors, and partially to the external origin, such as nutrition, radiation, temperature and stress. ... Various theories have evolved to improve our understanding of the ageing process so as to formulate strategies that enhance extension of life. The theories of ageing are classified based on the level at which the ageing mechanism is targeted: 1. Evolutionary theories, 2. Systemic theories, 3. Molecular and cellular theories ... Evolutionary theories state that ageing results from a decline in the force of natural selection. As evolution acts primarily to maximize reproductive fitness in an individual, longevity is a trait to be selected only if it is beneficial for fitness. Life span is, therefore, the result of selective pressures and may have a large degree of plasticity within an individual species as well as among species. ... In systemic theories, the ageing process is related to the decline of organ systems essential for control and maintenance of other systems within the organism. ... [Molecular and cellular theories] theories attempt to discern the mechanisms of ageing process at the cellular and subcellular levels."