In fabricating and testing of mirror surfaces for X-ray optics profile measurements with a height resolution of the order of one Å are required. This measurement accuracy is not provided by conventional interferometric test methods applied in the optical industry. Very powerfull instruments, however, are ac-interferometers which have been developed in recent years. AC-interferometers are capable of detecting height differences with a precision approaching one Å. The first part of this paper gives a survey of various ac-interferometers. Their main concepts and limitations will be discussed. In the second part a newly developed ac-technique will be presented. A step height measuring microscope built on this principle will be described. With a response time of 1 msec, this instrument is capable of detecting height differences smaller than 10 Å. This very practical measurement tool is primarily used to measure topographies of machined etched or sputtered surfaces. Application to slope mapping of spherical surfaces is feasible. Results of various measurements are presented.