Aging is the outcome of the progressive accumulation of different alterations in the body which accompanied with gradual decrease of the efficiencies of normal physiological functions and the capacity to maintain homeostasis that lead to the increase in disease probability and the death of people. The researchers have done different experiments especially on animal models for the perception of aging process, the longevity and the improvement of life quality. They have proposed more than 300 aging theories which overlap to some extent. In this article, we try to explain some of the most important theories with the emphasis on the epigenetic theory. The two most prominent biochemical theories of aging are free radical and mitochondrial theories. Normal aging is the result of the balance between damage and repair and these theories explain about the oxidative stress associated with the damages which result from reactive oxygen species (ROS), stress response signaling pathway and the antioxidant enzymes. According to the genetic theories, aging is the result of the controlled genetic program of the maturity and development. In addition, aging mitotic clock which guided by telomers, is important. The lengths of telomers which shorten after each cell division, can be regulated by many foods and telomeric epigenetic conditions. Furthermore, Aging influences not only by genes and mutations, but also by the environmental and epigenetic effects especially in the second half of life. The epigenetic changes include the alterations in gene expression without any changes in DNA sequences. Their biological and functional importance is accompanied with the loss of DNA global methylation, histone modification and gene promoter hypermethylation during aging process. To achieve to the dream of human for living healthier and longer, dietary manipulation with using the supplements and antioxidants, the understanding of a key role of the caloric restriction on longevity by the activity of Sirtuin proteins, the exercise and the perception of the environmental factors which affect epigenetic changes during aging process, are the most essential research issues in 21th century.