While noble metals such as platinum and its alloys are widely used for energy conversion catalysis in modern technology, biological systems, by controlling molecular environments and suppressing unfavorable oxidation reactions, manage to use abundant transition metals for the same purpose. To achieve this the reaction centers should not only be embedded in a chemically stable and electronically conducting matrix, which suppresses irreversible oxidation of the transition metals, but they also have to accommodate and transfer several electrons while sustaining a favorable redox potential. This is both theoretically and chemically a significant challenge but an important step towards mass application of energy catalysts in fuel cells or for water photo-electrolysis. This contribution will specifically address the oxygen electrode, both for reduction of oxygen to water and for photo-induced oxygen evolution from water and discuss experimental progress together with efforts to reach theoretical understanding. The role of non-linear synergetic interactions towards multi-electron transfer is emphasized and relevant mechanisms discussed on the basis of model catalysts. (c) 2006 Elsevier Ltd. All rights reserved.