There are a number of reasons for applying metallic or non‐metallic coatings to metal articles, the most common being to prevent the atmosphere coming into contact with the base metal and setting up corrosion. Others are (a) to provide a means of sacrificial corrosion; (b) to enable the electro‐chemical potentials of two metals in contact to be kept within certain limits; (c) to build up dimensions. In all these instances measurement of the thickness of the coating to a fair accuracy is necessary—if only to ensure that sufficient material has been applied to provide protection. One of the first commercial processes for coating one metal on another was the hot‐tinning of steel sheets and is still very much used. It is not surprising, therefore, that many of the methods of measuring coating thickness were first developed with the tinplate industry in mind. Tin is now applied in increasing quantities by electroplating processes and many other metals are used to form a protective coating on metallic articles. Measurements are usually affected to some extent by the quality {i.e. porosity, crystal structure, amount of alloying, etc.) of the coating and this must be taken into account when the final accuracy to be expected is considered.