The need for sensors to measure elements in molten metals is discussed from the viewpoint of energy saving and pollution control. Solid electrolytes are ideal materials for the development of sensors having good ionic conductivity, and;being refractory in nature. Potentiometric devices based upon stabilised zirconia and beta alumina type electrolytes are described for elements present at low concentrations. For high concentrations an amperometric device is described. However, there are many elements whose ions are not mobile in the ionic conductors and these can only be measured by covering the surface of the electrolyte with a compound which contains both the element to be measured and, also, the element whose ion is mobile in the electrolyte. Under some conditions, it is possible, by careful consideration of the stability of the compounds of the ion which is mobile in the electrolyte and the thermodynamics of the metallic solution, to select an electrolyte which will respond to the element without the need for an auxiliary coating.