With the development of accurate and precise techniques for quantifying sodium, potassium, calcium, and magnesium concentrations, a vast literature has been generated indicating that deficits and excesses of these cations in the body create many clinical challenges for the physician. Iron has received its share of attention as the most abundant trace metal in the body, primarily being utilized in the formation of hemoglobin. Zinc is the next most abundant trace element in the body, and evaluation of its role in the biochemistry and pathophysiology of disease is now stimulating a great deal of interest. Deficiencies of the other trace metals also are receiving increasing attention as attempts to evaluate their importance in human nutrition are being made. Studies were not considered reliable until technological advances made it possible, by atomic absorption spectroscopy, neutron activation analysis, and other techniques, to quantify these trace elements accurately and precisely.