Photochemistry is the study of the interaction of light with matter. This Primer describes the chemistry that follows the absorption of light, and explains the extraordinary influence of visible or ultraviolet light on chemical behaviour. After the absorption of light, a molecule may acquire completely different properties from those of its parent. The authors explain the role of light initiating this metamorphosis, the dependence of natural processes such as photosynthesis on photochemistry, the emission of radiation as fluorescence and phosphoresence, and current applications of photochemistry, which include photography, photopolymerization, photodegradable polymers, and the synthesis of organic chemicals. This book is an essential introduction to the intriguing field of photochemistry.