Chlorophyll biosynthesis in plant leaves is one of the most important biospheric processes. It enables production, renewal, and permanent maintenance of the main photosynthetic pigment required for effective photosynthesis and high productivity of plants. The key step in chlorophyll biosynthesis is photoreduction of its immediate precursor, protochlorophyllide. The purpose of this book is to summarize and discuss the data obtained in the studies on organization of the active pigment–protein complex, where the photoreduction of protochlorophyllide proceeds, the mechanisms of the intermediate reactions of this process and the pathways of functionally chlorophyll species formation. This book may be of interest for researchers and students in the field of photobiology and plant physiology.