The oxygen effect was observed as 1912 by Swartz. The ratio of doses administered under hypoxic to oxic conditions needed to achieve the same biological effect is called the oxygen enhancement ratio (OER). For low-LET radiation, such as photon radiation, the OER at high doses has a value of between 2.5 and 3, and the OER has a smaller value of about 2.5 or less at lower doses. The oxygen effect is large and important in the case of low-LET radiations. Radio-chemical reactions are generally believed to be the fundamental mechanisms underlying oxygen effects. Oxygen fixes the damage produced by free radical. In the absence of oxygen, damage produced by the indirect action may be repaired. The OER has been determined for a wide variety of chemical and biologic systems with different endpoints. For high-LET radiation such as heavy-ions, oxygen effect is very small. The oxygen-in-the-track hypothesis proposed to account for this effect, suggests that cells exposed to high-LET radiation exhibit an oxygenated microenvironment around the particle track, even when they are irradiated under anoxic conditions.