The adverse effects of high oxygen levels have been widely reported, and clinicians have struggled for many years to find the ideal balance between inspired oxygen levels and acceptable arterial oxygen saturation. However, when asked "what is an acceptable oxygen saturation," one is hard pressed to find a definitive answer. Permissive hypoxemia is a concept similar to the well-described strategy of permissive hypercapnia. It is a strategy that allows the arterial oxygen saturation to be less than normal in an attempt to minimize the amount of artificial support provided to the lungs by mechanical ventilation. It must be noted that this concept is predominantly based on physiology, as data in the medical literature are very limited. Permissive hypoxemia as an approach to acute lung injury remains controversial in the clinical setting.