The electron transfer step of photoelectrochemical reactions in light conversion devices designed on the basis of metal-oxide-semiconductor systems depends to a large extent on the complex molecular and electronic structure of the metal-electrolyte interface. As already mentioned, this step is the last of a series of consecutive transfer processes from the semiconductor to the electrolytes upon different interfaces. A comprehensive understanding of transfer processes at metal-electrolyte interfaces requires a description of the metal-electrolyte interface, beginning with the classical concepts, to arrive at a brief revision of the more complex quantum mechanical picture. Reactions of special technological interest, such as the hydrogen evolution and the reduction of CO2 in carbon fuels, will be treated.