Jn an interferometer which uses a reference surface the measured surface heights correspond to the difference between the test and reference surfaces. To determine the rms roughness of supersmooth surfaces accurately the effects of the reference surface roughness need to be removed. One technique for doing this involves averaging a number of uncorrelated measurements of a mirror in order to generate a reference surface profile which can then be subtracted from subsequent measurements so that they do not contain errors due to the reference surface. Th other technique provides an accurate rms roughness of the surface by taking two uncorrelated measurements of the surface. These two techniques for measurement of supersmooth surfaces are described and results of the measurement of a 0. 7 A rms roughness mirror are presented.