Photosystem II (PSII) is a multicomponent enzyme complex that catalyzes the light-induced water splitting to molecular oxygen, protons, and electrons. Photosystem II is located in the membranes of cyanobacteria, green algae, and plants. The crystallization of this complex from the thylakoid membranes poses great difficulties. The high sensitivity of photosystem II to light and radiation has an adverse effect on the crystal quality, as well as on the quality of X-ray diffraction data. This is the reason why the crystal structure of PSII from Thermosynechococcus elongates has as yet not been determined at high resolution. The optimization of the strategy for collecting X-ray diffraction data from PSII crystals has resulted in an increase in the resolution to 2.75 Å, which made it possible to determine the positions of ions and some water molecules playing an important role in the functioning of PSII.