A variety of techniques for measuring oxygen in normal and tumor tissue has been developed over the years in response to the realization that hypoxia is important in a number of pathophysiological conditions in normal tissues and in the response of tumors to radiation treatment. A review of the techniques for measuring tissue oxygenation is presented with an emphasis on clinical results. Histomorphometry, DNA strand break analysis (comet assay), oxygen microelectrodes and hypoxia markers are highlighted. Comparison among techniques is touched on and a short discussion of the possibility that hypoxia is not an independent variable in determining the outcome of radiation therapy is presented.