The origin of distortion in electron microscope projection lenses is discussed and the serious nature of its effect on the measurement of particle size distributions is pointed out. Methods of measuring distortion are described. By means of first-order theory it is shown to be possible to correct distortion by the use of a two element projection lens. The degree of correction obtainable is shown to be satisfactory for most practical purposes. A double gap projection lens polepiece and the correction of distortion obtained with it are described.