Hypoxic cells are present in rodent and xenografted human tumours and it has been known for a long time that the absence of oxygen in tumours is a factor of resistance against ionising radiation. The dose modifying role of oxygen (oxygen enhancement ratio) has been largely studied in experimental models. For pO2 values of 2 mmHg, the relative radiosensitivity of tumour cells is intermediate between the maximum sensitivity observed in air and the minimal one observed in hypoxia. The measurement of tumour pO2 in patients (polarographic technique) has demonstrated the presence of low values (< 10 mmHg) in many different tumour sites (ENT, uterine cervix, breast, melanoma, etc). In order to sensitise hypoxic tumours, imidazole have been used in patients, but most of the results were negative. New methods have been developed in the combination of bioreductive drugs of cytotoxic cells to radiotherapy. In this article, we will describe the clinical results obtained in patients with radiosensitising chemical agents.