Present knowledge of human bodily contents and concentrations in blood, urine, and hair of 11 essential trace elements and 17-22 nonessential inert or toxic trace elements is reviewed and summarized. Analyses of trace elements are applicable as diagnostic aids and indices for therapy in a number of clinical conditions. Techniques are not difficult, and analyses will probably become more or less routine for many diseases in which primary or secondary abnormalities are manifest. Trace elements play fundamental roles in human metabolism.