Although interferometers are emerging strongly from the laboratory into the shop floor, many users are totally oblivious to potential errors that inevitably creep into the measuring process. Even a certificate of calibration that may come with an interferometer does undoubtedly not guarantee a constant measurement uncertainty for all time. The components will age, change, become maladjusted and, hence, make it necessary to periodically re-evaluate the present uncertainty. However, conventional calibration techniques are scarcely applicable to interferometers since these techniques are typically based on calibration standards with consideration of a common rule of thumb which demands for the uncertainty of the calibration instrument to be one tenth of the specified measurement uncertainty of the instrument being calibrated. Thus, in case of interferometers, a calibration standard is required with a remaining uncertainty which is not only far beyond current metrological and productional capabilities but also strongly influenced by environmental circumstances. Since it is extremely difficult to determine or even to predict the systematic errors that occur in a real interferometer, a Virtual Interferometer for simulation of typical measurements and examination of accurately specified error influences is currently being developed. The Virtual Interferometer may also help to obtain the measurement uncertainty that occurs in an actual measurement task. This can be achieved by applying several calibration measurements with the real interferometer and comparing them with the simulated results. Now the actual measurement task can be simulated yielding virtual measurement uncertainty that can be used as an estimation for the actual measurement uncertainty.