As the terminal step in photosystem II, and a potential half-reaction for artificial photosynthesis, water oxidation (2H(2)O --> O(2) + 4e(-) + 4H(+)) is key, but it imposes a significant mechanistic challenge with requirements for both 4e(-)/4H(+) loss and O-O bond formation. Significant progress in water oxidation catalysis has been achieved recently by use of single-site Ru metal complex catalysts such as [Ru(Mebimpy)(bpy)(OH(2))](2+) [Mebimpy = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine; bpy = 2,2'-bipyridine]. When oxidized from to Ru(V) = O(3+), these complexes undergo O-O bond formation by O-atom attack on a H(2)O molecule, which is often the rate-limiting step. Microscopic details of O-O bond formation have been explored by quantum mechanical/molecular mechanical (QM/MM) simulations the results of which provide detailed insight into mechanism and a strategy for enhancing catalytic rates. It utilizes added bases as proton acceptors and concerted atom-proton transfer (APT) with O-atom transfer to the O atom of a water molecule in concert with proton transfer to the base (B). Base catalyzed APT reactivity in water oxidation is observed both in solution and on the surfaces of oxide electrodes derivatized by attached phosphonated metal complex catalysts. These results have important implications for catalytic, electrocatalytic, and photoelectrocatalytic water oxidation.