The schematic figure shows how a STM works: A metallic tip, which apex consists of a single atom, is moved with a small piezo tube in a separation of only few diameters of an atom (about 0,8 - 1 nm) above the sample surface. This tip is scanned above the sample surface. A voltage between tip and sample of a few mV to up to several V produces a tunneling current (nA), which is kept constant by a control unit in the feedback system. Because the current depends exponentially on the tip sample separation, an additional atom (adatom) on the surface causes the feed back loop to retract the tip. The control unit scans an area of the surface line by line while the height information is displayed as an image on the screen of a computer. The image reflects the electron density of the substrate surface and this density shows the geometrical arrangement of the atoms. Due to the very small tip sample separation, important technical problems considering the vibrational isolation as well as the exact movement of the tip in the sub nanometer scale have to be solved reliably. If several images are taken subsequently, the movement of diffusing atoms can be made visible.