The authors and publishers are pleased to present the thirty-first edition of Harper's Illustrated Biochemistry. The first edition, entitled Harper's Biochemistry, was published in 1939 under the sole authorship of Dr Harold Harper at the University of California School of Medicine, San Francisco, California. Presently entitled Harper's Illustrated Biochemistry, the book continues, as originally intended to provide a concise survey of aspects of biochemistry most relevant to the study of medicine. Various authors have contributed to subsequent editions of this medically oriented biochemistry text, which is now observing its 79th year. those of blood cells and plasma, which contained extensive content on metal ion adsorption and trafficking, especially of iron and copper. Since approximately a third of all proteins are metalloproteins, new Chapter 10 explicitly addresses the importance and overall pervasiveness of transition metals. Given the overlap with the topics of protein structure and of enzyme reaction mechanisms, new Chapter 10 now follows the three chapters on enzymes as the final chapter in Section II, now renamed Enzymes: Kinetics, Mechanism, Regulation,