Scanning tunneling microscopy and spectroscopy (STM/STS) were applied to study the nanostructural and nanoelectronic properties of ultrathin magnetic films, from individual adatoms and clusters (zero-dimensional systems) to three-dimensional bulklike islands. Complementary investigations of the nanomagnetic structure of ultrathin films were performed by magnetic force microscopy (MFM). Quantum magnetic structures were prepared by electron-beam lithography and nanosphere lithography and analyzed by STM/STS as well as by MFM. Magnetization switching of single-domain Co dots induced by the MFM tip is demonstrated, and its potential application for quantum magnetic storage is shown. Finally, tunneling spectroscopy performed with magnetic probe tips yields new information about the spin-resolved nanoelectronic properties of magnetic nanostructures.