The term isoenzyme is used to describe enzymically active proteins, catalysing the same reaction and occurring in the same species, but differing in certain of their physico-chemical properties. Although the occurrence of enzymes in multiple molecular forms has been recognized for several decades, the application of electrophoretic and, to a lesser extent, chromatographic techniques has led to rapid developments in recent years. Since the discovery of the heterogeneity of lactate dehy- drogenase, reported almost simultaneously by Vesel I and Beam and by Wieland and Pfleiderer in 1957, the multiple forms of this enzyme have become the subject of an extensive literature, and the techniques introduced have been applied to investigations of other isoenzymes. Isoenzymes, especially those of lactate dehydrogenase and alkaline phosphatase, have found many applications as diagnostic agents in clin- ical chemistry, while others, such as those of cholinesterase and glucose 6-phosphate dehydrogenase, exhibit a biological polymorphism of con- siderable interest in genetics. Recent additions to our understanding of the chemical structure of the lactate dehydrogenase isoenzymes have indicated their great potential in fields as diverse as immunology, em- bryology and comparative anatomy. These remarkable developments suggest that the time is opportune for present knowledge to be surveyed, and I have compiled this monograph in the hope that it will be found useful by biochemists, pathologists, clinicians, geneticists and others with interests in enzymology.