Water oxidation catalysis (WOC) is the acceleration (catalysis) of the conversion of water into oxygen and protons: 2 H2O → 4 H+ + 4 e− + O2 Many catalysts are effective, both homogeneous catalysts and heterogeneous catalysts. The oxygen evolving complex in photosynthesis is the premier example. There is no interest in generating oxygen by water oxidation since oxygen is readily obtained from air. Instead, interest in water oxidation is motivated by its relevance to water splitting, which would provide "solar hydrogen," i.e. water oxidation would generate the electrons and protons for the production of hydrogen. An ideal WOC would operate rapidly at low overpotential, exhibit high stability and be of low cost, derived from nontoxic components. Mechanistic and energetic principles Water is more difficult to oxidize than its conjugate base hydroxide. Hydroxide is stabilized by coordination to metal cations. Some metal hydroxides, those featuring redox-active metal centers, can be oxidized to give metal oxo complexes. Attack of water on metal oxo centers represents one pathway for the formation of the O-O bond, leading to dioxygen. Alternatively, the crucial O-O bond forming step can arise by coupling suitably positioned pairs of metal hydroxo centers. The molecular mechanism of the OEC has not been elucidated. The conversion of even metal hydroxo complexes to O2 requires very strong oxidants. In photosynthesis, such oxidants are provided by electron holes on porphyrin radical cations. For device applications, the aspirational oxidant is a photovoltaic material. For screening WOCs, ceric ammonium nitrate is a typical electron acceptor. Homogeneous catalysis Ruthenium complexes A number of ruthenium-aqua complexes catalyze the oxidation of water. Most catalysts feature bipyridine and terpyridine ligands. Catalysts containing pyridine-2-carboxylate exhibit rates (300 s−1) comparable to that of photosystem II. Work in this area has ushered in many new polypyridyl ligands. Cobalt and iron complexes Early examples of cobalt-based WOCs suffered from instability. A homogeneous WOC [Co(Py5)(H2O)](ClO4)2 operates by a proton-coupled electron transfer to form a [CoIII--OH]2+ species, which on further oxidation forms a CoIV intermediate. The intermediate formed reacts with water to liberate O2. The cobalt-polyoxometalate complex [Co4(H2O)2(α-PW9O34)2]10− is highly efficient WOC. Some iron complexes catalyze water oxidation. A water-soluble complex [Fe(OTf)2(Me2Pytacn)] (Pytacn=pyridine-substituted trimethyltriazacyclononane; OTf= triflate) is an efficient WOC. The concentration of the catalyst and the oxidant were found to be strongly affecting the oxidation process. Many related complexes with cis labile sites are active catalysts. Most complexes were found to undergo degradation