The crystal structure of photosystem II (PSII) at 1.9 Å resolution has been determined, providing detailed insights into the molecular mechanisms of water oxidation. PSII, a membrane-protein complex involved in photosynthetic water splitting, contains a Mn4CaO5 cluster responsible for catalyzing the reaction. The study reveals the precise arrangement of metal atoms, ligands, and water molecules within the cluster, with 5 oxygen atoms serving as oxo-bridges and 4 water molecules bound to the cluster. Over 1300 water molecules were identified in a single monomer, forming extensive hydrogen-bonding networks that may serve as channels for protons, water, or oxygen molecules. The high-resolution structure will facilitate a deeper understanding of PSII's functions, particularly in water splitting and O-O bond formation. The study also details the structure and function of chloride-binding sites, chlorophylls, β-carotenes, plastoquinones, and lipids, contributing to a comprehensive view of PSII's molecular architecture and mechanisms.The crystal structure of photosystem II (PSII) at 1.9 Å resolution has been determined, providing detailed insights into the molecular mechanisms of water oxidation. PSII, a membrane-protein complex involved in photosynthetic water splitting, contains a Mn4CaO5 cluster responsible for catalyzing the reaction. The study reveals the precise arrangement of metal atoms, ligands, and water molecules within the cluster, with 5 oxygen atoms serving as oxo-bridges and 4 water molecules bound to the cluster. Over 1300 water molecules were identified in a single monomer, forming extensive hydrogen-bonding networks that may serve as channels for protons, water, or oxygen molecules. The high-resolution structure will facilitate a deeper understanding of PSII's functions, particularly in water splitting and O-O bond formation. The study also details the structure and function of chloride-binding sites, chlorophylls, β-carotenes, plastoquinones, and lipids, contributing to a comprehensive view of PSII's molecular architecture and mechanisms.