Roughness and general surface topography of polished Si wafers were systematically studied on an angstrom to nanometer scale by scanning tunneling microscopy (STM). Evaluation of a large number of STM images by using a simple classification scheme assures statistically relevant results without the disadvantages and loss of information connected with averaging the rms roughness or similar methods. For the flat parts of the surface, a rms roughness of 1.2–1.8 A˚ is found, comparable to that derived from light scattering and diffraction measurements on similar surfaces. Significant amounts of the surfaces were found to exhibit more pronounced structures, contrasting results of other techniques. The reliability and applicability of STM measurements on technical surfaces, on an angstrom to nanometer scale, is discussed in the light of these results.