A new magnetic imaging technique, i.e., spin-polarized scanning tunneling microscopy, is presented. The technique is based on the tunneling magneto resistance (TMR) effect between a ferromagnetic tip and a ferromagnetic sample. By periodically changing the magnetization of a magnetically soft tip in combination with lock-in technique, topographic and spin-dependent parts of the tunneling current are separated and the topography and the magnetic structure of the sample can be recorded simultaneously with high resolution. Besides magnetic imaging, dynamic effects like domain wall mobility or the magnetic susceptibility can be studied locally with the double frequency response in the limit of soft magnetic materials or strong stray fields of the tip. We studied the closure domain structure of Co(0001) with high resolution and found surprisingly narrow sections of the wall of 1.1 nm width, over an order of magnitude less than previously observed in bulk Co. The ultra narrow domain walls are explained on the basis of a simple micromagnetic model which predicts a wall width of 1.5 nm. Further, measurements of the TMR versus the tunneling voltage and the tip-to-sample distance as well as the study of the infuence of a nonmagnetic Au layer on the TMR effect give deeper insight into the mechanisms of spin-polarized tunneling.