This book describes the occurrence, chemistry, and measurement of chemiluminescence, a phenomenon that together with the use of very sensitive light detectors has led to a number of exciting biomedical applications and new concepts about disease processes. Among the topics discussed by the author are the principles of using the reactions from fireflies and luminous bacteria to measure enzymes and metabolites, calcium-sensitive proteins from luminous jellyfish to measure changes in calcium inside living cells, and the measurement of oxygen radicals and the replacement of radioactive labels in immunoassay by chemiluminescent compounds. The author also discusses the future applications in research and clinical laboratories.