This book focuses on a variety of photochemical reaction processes in the crystalline state. The crystalline state reaction is a new category of solid state reaction, in which a reaction occurs with retention of the single crystal form. The whole reaction processes were observed directly by X-ray and neutron diffractions. In this book, not only the structures of metastable intermediates, such as radicals, carbenes, and nitrenes, but also the unstable species of photochromic compounds and photo-excited structures are shown with colored figures of the molecular structures, with more than 200 figures. The book is an indispensable resource not only for organic, inorganic and physical chemists but also for graduate students, as it furnishes more than 300 references.