We present static measurements of the domain structure and dynamic results on domain wall motion and local susceptibility obtained by spin-polarized scanning tunneling microscopy. The topography and the magnetic structure of the sample are recorded simultaneously with down to 10 nm resolution. With this technique, domain wall movement on Co(0001) is studied in situ. In some cases, the magnetization of the sample is locally influenced by the stray field of the tip. Measuring higher harmonics in the tunneling current allows one to quantify this influence and measure magnetic susceptibilities on similar scales.