Present methods of contour etching used by aircraft manufacturers in England and America are described and compared. Experiments to develop a production technique are described and factors controlling the process and product are discussed. There is no significant change in the tensile strength even when some 80 per cent of the original thickness is removed. It is doubtful if the etching process 'per se' reduces fatigue strength and the difficulties of making an analysis of effects on a clad material are discussed. The considerable advantages of the process in the production of aircraft are shown in some detail.