Publisher Summary A trace element is taken to mean an element that constitutes less than one per cent of the wet weight of biological material. Carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulphur are major elements of vital importance in biology. Trace elements are classified as inessential or essential according to whether an organism can grow and complete its life cycle in their absence. The functions of trace elements can be classified as inorganic/structural, electrochemical, catalytic, and miscellaneous or unknown. Examples of inorganic/structural functions include (1) apatite or calcium phosphate in vertebrate bone and tooth, (2) the barium sulphate crystals, which may act as gravity sensors in Loxodid cells, (3) magnetite (triferric tetroxide) crystals, which act as magnetic field sensors in bacteria and bees, and (4) silica or unknown derivatives, which may help to stiffen collagen. The essential trace elements Ca, Cl, Co, Cu, Pe, Mg, Mn, Mo, Ni, Se and Zn form compounds with proteins that have a wide range of catalytic functions.