We present an overview of spin-polarized scanning tunneling microscopy (Sp- STM). As in STM, the electron density near the sample surface can be imaged. In addition, Sp-STM allows us to map the spin polarization. Thus, information on the magnetic configuration of the sample surface can be gathered. Three imaging modes are currently being used: the constant-current mode, the spectroscopic mode, and the differential magnetic mode. The principles of the three modes are explained, and their advantages and limitations are discussed in the framework of imaging ferromagnetic and antiferromagnetic surfaces of bulk materials and thin film systems. Further, two approaches for controlling the spin direction of the tip apex, i.e., the sensitive spin component, are discussed. Surface or interface magnetic anisotropies at the tip apex may be used to align the axis of sensitivity or alternatively, the shape anisotropy of the whole tip may determine the spin direction. Finally, it is demonstrated that Sp-STM can be used beyond magnetic imaging. Valuable information on the spin resolved electronic structure or on the fundamental processes of spin-polarized tunneling may be obtained.