Tumour hypoxia is thought to contribute to some failures of radiotherapy to achieve local control. Polarographic measurements of tumour oxygenation have been shown to predict clinical response to radiotherapy and patient survival. Hypoxia is also involved in many common types of normal tissue morbidity. However, at present there is no widely used method of measuring hypoxia in the clinic, or for individualizing therapy on the basis of tumour or tissue oxygenation. The bioreductive metabolism of 2-nitroimidazoles provides a way of labelling hypoxic cells in vivo and a variety of isotopic labels have been proposed for the non-invasive detection of bound metabolites of these markers. Several 2-nitroimidazoles with immunologically identifiable side-chains have been described and conventional immunostaining procedures can be used to locate their metabolites, bound to hypoxic cells in histological sections. Use of fluorescent immunoreagents allows flow cytometric assessment of hypoxia and multiple colour fluorescent staining allows hypoxia to be correlated with other markers on a cell by cell basis. 2-Nitroimidazole hypoxia markers show considerable promise for clinical use in diagnosing hypoxia and their use could allow rational application of hypoxia-related therapies to those patients most likely to benefit from them.