The first edition of this book covered the basic treatment of the enzyme reaction using the overall reaction kinetics and stopped-flow method, the general properties of protein and cofactors, the control of enzyme reaction, and the preparation of enzyme protein. These topics are the basis of enzyme research and thus suitable for the beginner in the field. The second edition presents the cofactors produced via the post-translational modification of the enzyme’s active site. These cofactors expand the function of enzymes and open a new research field. The carbonyl reagent phenylhydrazine and related compounds have been useful in finding some of the newly discovered cofactors and thus have been discussed in this edition. The topic of the control of enzyme activity through the channel of substrates and products in polyfunctional enzymes has also been expanded in this book.