Hypoxia, a condition of reduced partial pressure of oxygen in tissue, is an important determinant of poor tumor response to radiation treatment. Many invasive and non-invasive methods and approaches have been investigated to detect tumor hypoxia for response prediction and to facilitate modulation of radiation treatment. In this review we discuss the biological consequences of tumor hypoxia, methods of measuring regional tumor hypoxia with positron emission tomography (PET) tracers and applications for radiation oncology.