Photosystem II (PSII) is a membrane-protein complex consisting of 17 trans-membrane subunits and 3 peripheral, extrinsic subunits with a total molecular mass of 350 kDa for a monomer. PSII performs a series of light-induced electron transfer reactions, leading to the conversion of light energy into biologically useful chemical energy, coupled with this is the splitting of water and generation of molecular oxygen. Both the chemical energy converted and molecular oxygen generated by PSII are indispensible for sustaining life on the earth; thus PSII is an extremely important protein complex. We have succeeded in crystallizing the PSII complex at a resolution of 1.9 Å, and analyzed its structure. Here we describe the structure of PSII, in particular the Mn₄CaO₅-cluster, which is the catalytic center of water-splitting, analyzed at the atomic resolution. Based on these, we discuss the possible mechanisms of light-induced water-splitting and its implications in artificial photosynthesis.