Cancer development in humans and animals may be caused by environmental factors. It has been estimated that approximately 80% of human tumors are generated by exposure to environmental carcinogens. The carcinogens may initiate or induce progression of tumors in several ways. Cellular senescence is a natural barrier used by cells to respond to stress. The molecular analysis of immortal clones shows alterations, either structural or epigenetic, in the genes involved in cellular senescence. It is thought that these alterations are caused directly by mutating or methylating the genes involved in cellular senescence. Therefore, understanding cellular senescence and how it can be modified by environmental carcinogens may be essential to control the increase in cancer prevalence.