The present book might be regarded as a sequel to my previous work, Bioinorganic Chemistry: An Introduction (Allyn and Bacon, 1977). The latter is essentially a collection of chemical and physical data pertinent to an understanding of the biological functions of the various elements and the proteins dependent on them. The ten years since its publication have seen an enormous increase in research activity in this area, hence of research papers. A number of monographs and review series on specific topics have also appeared, including the volumes in the series of which the present volume is a part. Nevertheless, a gap has developed between the flood of information available at a detailed level (papers and reviews) and a general description of the underlying principles of biofunctions of the elements as presently conceived. It is hoped that this book will help bridge this gap and at the same time provide an overview of the entire Biochemistry of the Elements series. Specifically, the work attempts to focus on "why" questions, especially, "Why has an element been chosen by organisms for a specific biofunction?" and "Why does an element behave the way it does in biological systems?" It therefore complements my 1977 book and, together with Laboratory Introduction to Bio-Inorganic Chemistry (E. -I. Ochiai and D. R. Williams, Macmillan, 1979), completes a trilogy on the topic of bioinorganic chemistry. This book consists of five parts. Two chapters constitute Part I.