Photosystem II (PSII) is a membrane protein complex which functions to catalyze light-induced water oxidation in oxygenic photosynthesis. Through the water-splitting reaction of PSII, light energy is converted into biologically useful chemical energy, and molecular oxygen is formed which transformed the atmosphere into an aerobic one and sustained aerobic life on the Earth. The PSII core complex from cyanobacteria consists of 17 transmembrane subunits and 3 extrinsic subunits with a total molecular mass of approximately 350 kDa per monomer, and PSII exists predominately in a dimeric form in vivo. This chapter describes the purification procedures leading to highly pure, homogenous, and highly active PSII core dimers from a thermophilic cyanobacterium, Thermosynechococcus vulcanus (T. vulcanus), that are used for successful crystallization and diffraction at atomic resolution. The purity and homogeneity of the PSII dimers thus obtained are characterized by absorption spectra, low-temperature fluorescence spectra, SDS-PAGE, clear native PAGE, blue native PAGE, gel filtration chromatography, and oxygen-evolving activity measurements. Finally, high-quality crystals obtained from the purified PSII dimers are shown.