The velocity of aging is rather different in various species, and even in various tissues and cells of the same individual. Both genetic and environmental factors affect aging process. It is evident that life expectancy mainly relates to environment, while maximum life-span of a species more depends on its genetic background. Poor environment possibly affects genes or their products and then influences the process of human senescence. As aspect of genetic causes, aging is not totally depends on one gene, but rather on the interaction of networks of activated or repressed genes and their products. DNA (especially, mitochondrial DNA) is not as stable as previously conceived. The stability of genetic substance, including genes, could be affected by stresses from external or internal environment, which are particularly induced by harmful substances, such as reactive oxygen species, leading to accelerate aging process.