In this article we have presented the basic principles which underpin practical electro-etching using simple equipment and procedures. Although the real world can be more complex, generally the electrolytic cells used for electro-etching, particularly those using the metal/ same metal sulphate electrolyte combination, are well behaved and can be adequately explained by the present simplified treatment. The use of sodium chloride solution as an electrolyte offers, in principle, the possibility of electro-etching aluminium as well as zinc, copper, iron and steel. However its use is not so straightforward and is best left to the experienced electro-etcher with sufficient knowledge to carry out the electro-etching safely.