Abstract: Abstract The history of enzymes and of the ideas concerning them in relation to physiologic processes is reviewed. The importance of enzymes in pathologic processes, and the great usefulness of their determination in body fluids in relation to diagnosis, prognosis and in the assessment of therapy is pointed out. The enzymes most commonly used in diagnosis are listed, the methods for their determination briefly indicated, and the range of activities in normal blood plasma and other fluids stated. The clinical conditions in which abnormalities have been found are given for each enzyme, together with some indication as to the size of the change and its direction.