Understanding the pathophysiology of the optic nerve head (ONH) requires precise knowledge of the oxygenation of this tissue. Previous investigations used methods based on oxygen sensitive microelectrodes to measure the partial pressure of oxygen in the ONH tissue. These microelectrodes are inserted into the eye through the pars plana. New methods are now being developed to measure the intravascular Po2 in the ONH. They are based on the quenching of phosphorescence of a porphyrin dye by oxygen. Although these methods require an intravenous injection of this dye, the eye is left untouched. This article reviews these noninvasive methods.