The appearance of the first review in 1965 [1] and the first monograph in 1968 [2] on chemiluminescence demonstrated the extent of the phenomenon of light emission from the reaction of organic compounds in solution. Since then the num ber of chemiluminescent compounds has greatly increased, although the advan ces in theory and, more recently, applications are probably more significant. The present work is written by two authors who, together with E. H. White, helped to bring the study of chemiluminescence into the modern era. However many investigators are making contributions to the subject, even if the number of enthusiasts still remains small. It is not our intention to write an exhaustive account of chemiluminescence, still less of bioluminescence, and we have concentrated on making the landmarks in the area familiar to a readership outside the circle of specialists. The emphasis is on the range of organic compounds showing light emission with very little description of the relatively few inorganic or the more numerous biological examples which have been discovered. We hope that some of the excitement of the striking demonstrations of chemiluminescence which can be made appears in the text, albeit in the form of intellectual satisfaction and interest. We thank Prof. Dr. J. Stauff, Frankfurt for his generous advice and his critical comments. The chapter dealing with Peroxy-oxalate chemiluminescence has been commented up on critically by Dr. M. M. Rauhut, Stamford, Connecticut which we gratefully acknowledge.