We present an overview of the technique of spin-polarized scanning tunneling microscopy (Sp-STM) and its application to high-resolution magnetic imaging. In STM, the electron density near the sample surface is imaged. Additionally, Sp-STM allows a mapping of the spin polarization of the electronic density, which is related to the magnetic configuration of the sample. Two primary imaging modes of Sp-STM are currently in use: the spectroscopic mode and the differential magnetic mode. The principles of the two modes are explained in the framework of imaging ferromagnetic nanostructures and antiferromagnetic surfaces. The advantages and drawbacks of the two approaches are discussed, and the strength of Sp-STM to map even complex spin structures on the nanometer scale is illustrated.