J.B.S. Haldane will be long remembered for his many contributions to man's knowledge of his world and of himself. Some of the most valuable of these contributions are contained in this classic work on the chemistry of enzymes, originally published in 1930. The book sheds new light on research possibilities that have heightened relevance today. In the preface to this edition, written just a few months before his death, the author has pointed out the importance of the modern enzymologist's awareness of what was known about his subject in the recent past. He describes briefly the major advances in enzymology in the last thirty years and has urged continued and intensified research in specific areas. The result is a fascinating historical work by a great scientist with specific relevance for those in the field today.