PURPOSE: Tumor hypoxia plays a fundamental role in tumor progression and treatment resistance. Recent evidence that hypoxia also influences the regulation and transcription of various genes involved in malignant growth and metastases, and promotes a more aggressive tumor phenotype makes its diagnosis even more important. PROCEDURES: The evidence for the biology of hypoxia in tumors, and imaging of hypoxia with different technologies was reviewed through literature review and Medline searches, and clinical studies with 18F-fluoromisonidazole (FMISO) Positron Emission Tomography (PET). RESULTS: Until recently, determination of the level of tumor oxygenation was only possible using invasive methods that limited its clinical application. Imaging techniques that have shown promise in assessing hypoxia include magnetic resonance imaging and spectroscopy, single photon emission computed tomography (SPECT) and PET. Quantitative hypoxia measurement with 18F-FMISO PET in patients with malignant gliomas and lung cancer have demonstrated intratumoural hypoxia and dissociation of glucose metabolism from hypoxia in some cases, indicating the complex nature of cellular metabolic response to stress. CONCLUSION: The emerging role of therapies that have improved efficacy in hypoxic conditions, and recent advances in the ability to noninvasively measure in vivo intratumoral hypoxia with functional imaging has renewed interest in the clinical measurement of tumor hypoxia and its impact on cancer treatment.