Artificial photosynthesis is one of the promising clean-energy-providing systems of the future. Development of an efficient catalyst for water oxidation to evolve O2 is indispensable for the construction of artificial photosynthetic devices. The photosynthetic oxygen evolving complex (OEC) composed of a tetranuclear manganese cluster serves as an excellent example of an efficient catalyst for water oxidation. Recently, significant progress has been reported in the development of manganese complexes as OEC models. The molecular aspects and activities of these manganese complexes toward water oxidation catalysis were reviewed in terms of homogeneous and heterogeneous catalysis systems. These could provide hints to design an efficient catalyst, as well as a key model reaction to understand the O2 evolution mechanism in photosynthesis.