Since the invention of the Scanning Tunneling Microscope (STM) [4.1] the question arose whether this technique, besides its ability to probe the Local Electronic Density of States (LDOS) [4.2], can also be made sensitive to the electron spin, thus offering the opportunity to study magnetic structures down to the atomic scale. Spin-polarized tunneling was already discovered in the seventies when planar tunnel junctions, either ferromagnetoxide-superconductor [4.3] or ferromagnet-semiconductor-ferromagnet junctions [4.4], were studied. However, it took until 1990 for the first successful demonstration of vacuum tunneling of spin-polarized electrons with the STM [4.5].