This book is based on experience gained teaching molecular biology to students at Harvard College and Harvard Medical School. It offers and integrated and self-contained approach to one of the most fascinating areas of biology and chemistry -- the study of enzymes, the molecular machines that are responsible for carrying out the activities of living cells. The book begins simply and gradually becomes more complex as discoveries are introduced that represent the frontiers of modern chemistry. The whole of the material is arranged as a story -- it leads from the discovery of enzymes to an exploration of their molecular mechanism of action to an understanding of their role in carrying out the physiological precesses of the body.