Recently there has been increased interest in the measurement of the microtopography of optical surfaces. The information generated may be used either to predict the scattering characterisitics of a particular surface or to relate surface quality to damage thresholds. Previously, surface profiles have been measured by a variety of techniques including electro-mechanical profilometry and multiple beam interferometry. Surface profiles may also be measured using a scanning Fizeau interferometer. The instrument is similar to conventional Fizeau interferometers, with the exception that the interference pattern is temporally modulated by vibrating the test sample. A detector scanned across the modulated interference pattern produces an ac signal, with a phase that is directly proportional to the profile of the surface under test. An instrument based on this principle has been constructed and is capable of generating profiles of surfaces with rms roughnesses of approximately 20 A. The advantages and limitations of the instrument are discussed. Experimental results are presented.