The book "Oxygen Evolving Complex" offers a comprehensive analysis of the oxygen-evolving complex (OEC), also known as the water-splitting complex. This enzymatic component plays a crucial role in the photooxidation of water during the light reactions of photosynthesis. The work is based on the widely accepted theory from 1970, which states that the complex can exist in five states, from S0 to S4. It explains how photons captured by photosystem II move the complex through these states, with S4 being unstable and reacting with water to produce free oxygen. Despite extensive research supported by flash experiments, EPR, and X-ray spectroscopy, the functional mechanism of the OEC remains incompletely understood. This specialized book is aimed at readers interested in the biochemical processes of photosynthesis and the associated enzymatic mechanisms.