Enzymatic reduction of oxygen is an important process in many living organizations; it is the key final step in the overall cycle of extracting energy from biofuels via oxidation. It is also a key step in the operation of technological contrivances, such as biosensors, biofuel cells, and biosupercapacitors. Issues like generation of harmful reactive oxygen species must be dealt with in living systems, as well as in technological applications. This article is focused on enzymatic electrocatalysis in the oxygen reduction reaction. Both direct-electron- and mediated-electron-transfer reactions are considered, nevertheless with some focus on the mediator-free approach. The active sites of enzymes, especially oxidoreductases, are intrinsically highly active. However, it appears that few enzymes can be fruitfully used for four-electron electroreduction of oxygen directly to water. Moreover, owing to limitations in oxygen availability and diffusion, oxygen bioelectroreduction has emerged as a rate-limiting step in overall fuel cycle processes and poses challenges in potential applications. Inadequate stability of redox enzymes is also an issue which is not often addressed. The potential and challenges for technological applications have been a driving force for research, but insight gained into biocatalyst structure/function is also mentioned where appropriate.