We reviewed research on retinal oxygen (O2) distribution and use, focusing on O2 microelectrode studies in animals with circulatory patterns similar to those of humans. The inner and outer halves of the retina are different domains in terms of O2. Understanding their properties can suggest mechanisms of and therapies for retinal diseases. Inner retinal PO2 averages about 20 mm Hg. Effective O2 autoregulation of the retinal circulation ensures that inner retinal PO2 is relatively uninfluenced by systemic hypoxia and hyperoxia and increased intraocular pressure in healthy animals. Failures of the retinal circulation lead to tissue hypoxia that underlies the vasoproliferation in diabetic retinopathy and retinopathy of prematurity. Choroidal blood flow is not regulated metabolically, so systemic hypoxia and elevated intraocular pressure lead to decreases in choroidal PO2 and photoreceptor O2 consumption. The same lack of regulation allows choroidal PO2 to increase dramatically during hyperoxia, offering the potential for O2 to be used therapeutically in retinal vascular occlusive diseases and retinal detachment.