Human solid tumours are considerably less well oxygenated than normal tissues. This leads to resistance to radiotherapy and anticancer chemotherapy, as well as predisposing to increased tumour metastases. However, tumour hypoxia can be exploited in cancer treatment. One such strategy is to use drugs that are toxic only under hypoxic conditions, and the first drug of this class to enter clinical testing, tirapazamine, is showing considerable promise. The second way to exploit hypoxia is to take advantage of the selective induction of the transcription factor hypoxia-inducible factor 1 (HIF-1) under hypoxic conditions; gene therapy strategies based on this are in development.