Scanning tunneling microscopy (STM) measurements of X-ray mirrors are presented. The X-ray mirrors are 0.3 mm thick dip-lacquered aluminum foils coated with gold by evaporation, as well as state-of-the-art polished surfaces coated with gold, platinum, or iridium. The measurements reveal that the surfaces consist of islands with different topographic features. The microroughness is found to be in the range from 7 to 15 A, and the characteristic length scale for this microroughness is estimated to be between 0.03 and 0.06 micron. For the thin foil mirrors, it is found that the microroughness depends on the thickness of the gold layer. The roughness is smallest (about 7 to 9 A) for gold layers between about 100 and about 250 A, and it becomes significantly greater (about 10 to 15 A) for gold layers thicker than about 350 A. With a few exceptions, the STM measurements agree well with recent X-ray studies. The results can be used as a guide when selecting the best coating process in the production of X-ray mirrors.