The Scanning Tunneling Microscope (STM) was introduced by G. Binnig and W. Rohrer at the IBM Research Laboratory in 1982 which was honoured by the Noble Prize in 1986. It has become widely used as an important instrument for real space analysis in surface science and provides images of solid surfaces with unprecedented resolution. The basic idea is to bring a fine metallic tip in close proximity (a few Å) to a conductive sample. The operation is based on the so-called tunneling current, which starts to flow when a sharp tip approaches a conducting surface at a distance of approximately one nanometer. By applying a voltage (U 4V) between the tip and the sample a small electric current (0.01nA-50nA) can flow from the sample to the tip or reverse, although the tip is not in physical contact with the sample. This phenomenon is called electron tunneling . The tip is mounted on a piezoelectric tube, which allows tiny movements by applying a voltage at its electrodes. Thereby, the electronics of the STM system controls the tip position in such a way that the tunneling current and, hence, the tip-surface distance is kept constant, while at the same time scanning a small area of the sample surface. The exponential dependence of the tunneling current on the tip to sample distance results in a high vertical resolution. By scanning the tip across the surface and detecting the current (one can also use the current as a vertical positioning signal for the tip; a map of the surface can be generated with a resolution in the order of atomic distances). It has to be mentioned that the image cannot just be interpreted as a topographic map as the tunneling current is influenced by the lateral and vertical variation of the electronic state density at the surface. The lateral resolution is about 1Å whereas a vertical resolution up to 0.01Å can be achieved. The STM can be used in ultra high vacuum, air or other environments. A typical piezoelectric material used in STMs is PZT (Lead Zirconium Titanate). Determination of surface structures is one of the most important applications of the STM.