A number of powerful chemical compounds that modify radiation effects have been discovered and tested both in the laboratory and clinically over the past 25 years. There are four major classes of compounds: aminothiol radio-protectors which act on well vascularized euoxic cells and concentrate in tissues such as skin, gut and marrow; nitromidazole radiosensitizers which act on hypoxic tumor cells; pyrimidine analogues which are incorporated into the DNA of cycling cells and cause radiosensitization; and cancer themotherapy agents which, in addition to their ability to kill tumor cells directly, also may sensitize tumor and normal cells to radiation. The mechanism of action, experimental activity, and clinical results or the potential for each of these agents are reviewed.