The remarkable development of molecular biology has had its counterpart in an impressive growth of a segment of biology that might be described as atomic biology. The past several decades have witnessed an explosive growth in our knowledge of the many elements that are essential for life and maintenance of plants and animals. These essential elements include the bulk elements (hydro­ gen, carbon, nitrogen, oxygen, and sulfur), the macrominerals (sodium, potas­ sium, calcium, magnesium, chloride, and phosphorus), and the trace elements. This last group includes the ultra trace elements and iron, zinc, and copper. Only the ultratrace elements are featured in this book. Iron has attracted so much research that two volumes are devoted to this metal-The Biochemistry of Non-Heme Iron by A. Bezkoravainy, Plenum Press, 1980, and The Biochemistry of Heme Iron (in preparation). Copper and zinc are also represented by a separate volume in this series. The present volume begins with a discussion of essentiality as applied to the elements and a survey of the entire spectrum of possible required elements.