In scanning tunneling microscopy (STM; see Scanning Tunneling Microscopy), the electron charge is used as information carrier to image the topography of a sample surface. The small tunneling current between the tip and the conductive surface is used as a feedback parameter to move the tip on lines of constant tunneling current. In first approximation, these lines correspond to lines of constant charge density of the sample surface probed by the tip apex (Tersoff and Hamann 1983). For ferromagnetic or antiferromagnetic materials, the charge density is spin split into majority and minority states. A net-spin polarization is present in the atoms and the individual atoms carry a magnetic moment. In spin-polarized STM (Sp-STM), the spin of the tunneling electrons is used as an additional information carrier to map the spin polarization of the sample surface. This way, Sp-STM allows one to image both the topography and the magnetic structure of a surface with nanometer-lateral resolution. Due to the strong localization of the tip-sample interaction, the resolution is superior to magnetic force microscopy (MFM; see Magnetic Force Microscopy), which uses the long-range magnetic dipole interaction.