Cell senescence is broadly defined as the physiological program of terminal growth arrest, which can be triggered by alterations of telomeres or by different forms of stress. Neoplastic transformation involves events that inhibit the program of senescence, and tumor cells were believed until recently to have lost the ability to senesce. It has now become apparent, however, that tumor cells can be readily induced to undergo senescence by genetic manipulations or by treatment with chemotherapeutic drugs, radiation, or differentiating agents. Treatment-induced senescence, which has both similarities with, and differences from, replicative senescence of normal cells, was shown to be one of the key determinants of tumor response to therapy in vitro and in vivo. Although senescent cells do not proliferate, they remain metabolically active and produce secreted proteins with both tumor-suppressing and tumor-promoting activities. Expression of tumor-promoting factors by senescent cells is mediated, at least in part, by senescence-associated cyclin-dependent kinase inhibitors such as p21(Waf1/Cip1/Sdi1). Clinical and preclinical studies indicate that expression of different biological classes of senescence-associated growth-regulatory genes in tumor cells has significant prognostic implications. Elucidation of the genes and regulatory mechanisms that determine different aspects of tumor senescence makes it possible to design new therapeutic approaches to improving the efficacy and to decreasing the side effects of cancer therapy.