Abstract: Cancers of the head and neck are a malignancy causing a considerable health burden. In head and neck cancer patients, tumor hypoxia has been shown to be an important predictor of response to therapy and outcome. Several imaging modalities can be used to determine the amount and localization of tumor hypoxia. Especially PET has been used in a number of studies analyzing this phenomenon. However, only few studies have reported the characteristics and development during (chemoradio-) therapy. Yet, the characterization of tumor hypoxia in the course of treatment is of great clinical importance. Successful delineation of hypoxic subvolumes could make an inclusion into radiation treatment planning feasible, where dose painting is hypothesized to improve the tumor control probability. So far, hypoxic subvolumes have been shown to undergo changes during therapy; in most cases, a reduction in tumor hypoxia can be seen, but there are also differing observations. In addition, the hypoxic subvolumes have mostly been described as geographically rather stable. However, studies specifically addressing these issues are needed to provide more data regarding these initial findings and the hypotheses connected with them