Bioluminescence, the production of light by animals and plants, has always excited the admiration of the layman and the wonder of the scientist. It is not surprising that an enormous literature dealing with the subject has grown up. A large part of this literature, however, is made up merely of reports that a certain animal is luminous, or records of especially brilliant phosphorescence of the sea. It is not the purpose of this book to deal with every phase of bioluminescence. We shall be concerned largely with the physical characteristics of animal light and the chemical processes underlying its production.