The subject of full-scale aeroplane experiments is one which I am sure is of interest to all concerned in aeronautical engineering. The interest of the present paper would be increased if it were possible to include in it detailed descriptions of actual experiments, numerical results, and a discussion of their bearing on design. It will be realised that this is impossible. With so much to choose from each of a number of experiments would deserve fully the time which I have to divide among them all, and a paper with this title can only be a survey of the whole field, dealing with certain general questions of importance.