Books dealing with the mechanisms of enzymatic reactions were written a generation ago. They included volumes entitled Bioorganic Mechanisms, I and II by T.C. Bruice and S.J. Benkovic, published in 1965, the volume entitled Catalysis in Chemistry and Enzymology by W.P. Jencks in 1969, and the volume entitled Enzymatic Reaction Mechanisms by C.T. Walsh in 1979. The Walsh book was based on the course taught by W.P. Jencks and R.H. Abeles at Brandeis University in the 1960's and 1970's. By the late 1970's, much more could be included about the structures of enzymes and the kinetics and mechanisms of enzymatic reactions themselves, and less emphasis was placed on chemical models. Walsh's book was widely used in courses on enzymatic mechanisms for many years. Much has happened in the field of mechanistic enzymology in the past 15 to 20 years. Walsh's book is both out-of-date and out-of-focus in today's world of enzymatic mechanisms. There is no longer a single volume or a small collection of volumes to which students can be directed to obtain a clear understanding of the state of knowledge regarding the chemicals mechanisms by which enzymes catalyze biological reactions. There is no single volume to which medicinal chemists and biotechnologists can refer on the subject of enzymatic mechanisms. Practitioners in the field have recognized a need for a new book on enzymatic mechanisms for more than ten years, and several, including Walsh, have considered undertaking to modernize Walsh's book. However, these good intentions have been abandoned for one reason or another. The great size of the knowledge base in mechanistic enzymology has been a deterrent. It seems too large a subject for a single author, and it is difficult for several authors to coordinate their work to mutual satisfaction. This text by Perry A. Frey and Adrian D. Hegeman accomplishes this feat, producing the long-awaited replacement for Walsh's classic text.