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For certain irradiations of free living things and cells, there is an increased radiosensitivity in the presence of oxygen than in anoxic (0% O2), or hypoxic conditions that where the oxygen tension is less than 1% of atmospheric pressure (i.e., <1% of 101.3 kPa, 760 mmHg or 760 torr). This so-called oxygen effect has particular importance in external beam radiotherapy where the killing of tumour cells with photon and electron beams in well oxygenated regions can be up to three times greater than in a poorly vasculated portion of the tumour. The oxygen effect is also relevant to hypoxia conditions present in the normal physiology of stem cell niches such as the endosteum adjacent to bone in bone marrow and the epithelium layer of the intestine. In addition, there are non-cancer diseases where oxygenated tissues can become hypoxic such as in stenosed coronary arteries associated with cardiovascular disease. Holthusen (1921) first quantified the oxygen effect finding 2.5 to 3.0-fold less hatching eggs of the nematode, Ascaris in oxygenated compared to anoxic conditions, which was incorrectly assigned to changes in cell division. However, two years later Petry first attributed oxygen tension as affecting ionizing radiation effects on vegetable seeds. Later, the implications of the effects of oxygen on radiotherapy were discussed by Mottram. A key observation regarding biological mechanisms proffered to explain the oxygen effect is that it is only observed if the gas is present at irradiation or within milliseconds, afterward. The best known explanation of the oxygen effect is the oxygen fixation hypothesis developed by Alexander in 1962, which posited that radiation-induced non-restorable or “fixed” nuclear DNA lesions are lethal to cells in the presence of diatomic oxygen. This hypothesis has not been confirmed experimentally. Recent hypotheses include one based on oxygen-enhanced damage from first principles. Another hypothesis posits that ionizing radiation provokes mitochondria to produce reactive oxygen species, which are leakage during oxidative phosphorylation that varies with a hyperbolic saturation relationship observed with the oxygen effect. The oxygen effect is quantified by measuring the radiation sensitivity or Oxygen Enhancement Ratio (OER) of a particular biological effect (e.g., cell death or DNA damage, which is the ratio of doses under pure oxygen and anoxic conditions. Consequently, OER varies from unity in anoxia to a maximum value for 100% oxygen of typically up to three for low ionizing-density-radiation (beta-, gamma-, or x-rays), or so-called low linear energy transfer (LET) radiations. Radiosensitivity varies most rapidly for oxygen partial pressures below ~1% atmospheric (Fig. 1). Howard-Flanders and Alper (1957) developed a formula for the hyperbolic function of OER and its variation with oxygen concentration, or oxygen pressure in air. Radiobiologists identified additional characteristics of the oxygen effect that influence radiotherapy practises. They found that the maximum OER value diminishes as the ionizing-density of the radiation increases (Fig. 2), from low-LET to high-LET radiations. The OER is unity irrespective of the oxygen tension for alpha-particles of high-LET around 200 keV/μm. The OER is reduced for low doses as evaluated for cultured mammalian cells exposed to x-rays under aerobic (21% O2, 159 mmHg) and anoxic (nitrogen) conditions. Typical fractionation treatments are daily 2 Gy exposures, as below this dose the so-called `shoulder` or repair region of the curve is encroached upon reducing the OER (Fig. 3). Parmar K, Mauch P, Vergilio JA, Sackstein R, Down JD. Distribution of hematopoietic stem cells in the bone marrow according to regional hypoxia. Proceedings of the National Academy of Sciences. 2007;104(13):5431-6.