The main objective of this report is to describe the key chemistry related to water oxidation catalysis by molecular transition metal complexes. The nature of a few mononuclear and polynuclear Ru complexes that are of interest from a mechanistic perspective is illustrated. Furthermore, the main intermediates proposed in their corresponding catalytic cycles is also discussed together with the existing parallel chemistry with Ir complexes. Finally, the potential applications of these water oxidation catalysts for light induced water splitting devices is also succinctly presented.