The article by G. M. J. Schmidt reviews the current research on solid-state photochemistry, focusing on four main areas: the topochemical postulate, the locus of the reaction, the understanding of organic molecule packing in crystals, and crystal engineering. The topochemical postulate suggests that the course of solid-state reactions and the stereochemistry of photodimers can be predicted from the configuration and geometry of monomer molecules in the crystal lattice. The locus of the reaction examines how the course of the reaction depends on crystal texture, such as dislocations, grain boundaries, and phase boundaries. The article also discusses the importance of understanding molecular packing principles to construct photoreactive or light-stable crystal structures and mixed crystals for synthetic purposes or energy-transfer studies. Finally, it outlines approaches to crystal engineering based on generalizations of packing modes of primary amides and dichlorophenyl derivatives, and their application to systematic solid-state photochemistry. The article highlights the historical development of solid-state photochemistry, from early studies on thermal reactions to more recent advancements in understanding the interplay between molecular packing and photochemical behavior.The article by G. M. J. Schmidt reviews the current research on solid-state photochemistry, focusing on four main areas: the topochemical postulate, the locus of the reaction, the understanding of organic molecule packing in crystals, and crystal engineering. The topochemical postulate suggests that the course of solid-state reactions and the stereochemistry of photodimers can be predicted from the configuration and geometry of monomer molecules in the crystal lattice. The locus of the reaction examines how the course of the reaction depends on crystal texture, such as dislocations, grain boundaries, and phase boundaries. The article also discusses the importance of understanding molecular packing principles to construct photoreactive or light-stable crystal structures and mixed crystals for synthetic purposes or energy-transfer studies. Finally, it outlines approaches to crystal engineering based on generalizations of packing modes of primary amides and dichlorophenyl derivatives, and their application to systematic solid-state photochemistry. The article highlights the historical development of solid-state photochemistry, from early studies on thermal reactions to more recent advancements in understanding the interplay between molecular packing and photochemical behavior.