Tumour hypoxia is known as one of the major factors that influence the response of head and neck cancer (HNC) to treatment in general and to radiotherapy in particular. Hypoxia occurs in tumours because of the particular characteristics of the tumour vasculature leading to impaired oxygenation of the cells. There are two major types of hypoxia: chronic or diffusion limited hypoxia and acute or perfusion limited hypoxia. They differ with respect to the way they originate and their temporal patterns, their dynamics being related to the so-called slow and respectively fast re-oxygenation. Since hypoxia has long time been recognised as one of the main reasons for the failure of treatment of HNC, several methods with different underlying principles have been developed for identifying the presence its presence in tumours, for visualising it and for quantifying its extent and severity. Among them, the Eppendorf histograph is still considered to be the golden standard as positron emission tomography (PET) and magnetic resonance imaging (MRI) are the most promising emerging techniques. There are several modalities for overcoming the resistance to treatment associated with tumour hypoxia including dose escalation based on functional imaging, the use of high linear energy transfer (LET) radiation, radiosensitisers and bioreductive drugs, cytotoxic hypoxic agents, and hyperbaric increase of oxygen supply, presenting various advantages and disadvantages and holding promises for improving the outcome of the treatment of hypoxic tumours in the near future.