SUMMARYThe expression used in the electron microscopical measurement of dislocation densities is ϱ=2N/Lt, whereNis the number of intersections between the dislocation lines in a TEM image,Lthe total length of a set of sampling lines superimposed upon that image (corrected for magnification) andtthe thickness of the thin foil containing the dislocation structure. During the estimation of ϱ it is therefore important to reduce errors in the measurements of all three variables. It is shown how calibration of the microscope magnification, careful consideration of diffraction conditions and use of the hollow cone and rocking beam facilities on the hybrid diffraction unit fitted to the Philips EM 400T can assist in increasing the speed and accuracy of the measurement procedur