The Phase Measuring Interferometer (PMI) technique has been adapted to the measurement of small scale features of very high quality (low scatter) optical surfaces. The method described has good vertical and lateral resolution over a wide range of fields. It is non-contacting, applicable to various materials and coatings, and can be extended to spherical and aspherical surfaces. The compact PMI device can be modified to provide in-situ measurements on both small and very large optical surfaces. Data, including rms surface roughness and surface contour maps, are presented for several surfaces. The nature and effect of the principal error sources are also discussed.