Abstract: Hypoxia in solid tumors is a major problem for radiotherapy, so great interest has been given to the development of radiosensitizers which can enhance tissue response to radiation. Nowadays, research on radiosensitizers includes conventional nitroimidazoles, hypoxic cytotoxins, nitric oxide donor agents, allosteric effectors of haemoglobin, metalloporphyrins and so on. They can either increase oxygen delivery to the tumor or selectively kill the hypoxic cells. Though many compounds of different types have been synthesized, none of them can be used clinically because of the complexity of tumor microenviroment.