The role of oxygen as an enhancer of the biological effect of ionising radiation is crucial for radiotherapy. Most of the solid tumours have an inadequate vasculature (neo-angiogenesis) leading to hypoxic areas, favoured by anaemia. Many clinical approaches have been used to overcome this problem with oxygen breathing, radiosensitizing drugs or anaemia correction. All these parameters will be developed together with the methods available for measuring tumour oxygenation and the biological consequences of anaemia and hypoxia on tumour physiology.