Scanning Tunneling Microscopy (STM) is a versatile surface characterization tool that is used in this thesis to observe both surface and subsurface nanostructures. With STM one is able to observe subsurface noble-gas-¯lled nanocavities in a Cu(001) substrate, when the nanocavities are positioned several nanometers below the surface. In essence, the main principle is that electrons are injected by the STM tip into the substrate and are re°ected by the nanocavity back to the surface of the substrate, where they are detected by the STM due to changes in the tunneling current. Quantum well states are thus formed between the top facet of the nanocavity and the surface of the substrate. The interference of injected and re°ected electrons leads to complex conductance patterns imaged by the STM for Ar, He, and Ne-¯lled nanocavities. These patterns are strongly in°uenced by the band structure of the Cu(001) substrate, that induces hot electron focusing in the h101i directions. Furthermore, STM images with atomic resolution reveal a remarkable apparent c2£2 superstructure spreading in narrow beams in the h100i directions at the Cu(001) surface. This apparent superstructure is a Moire pattern, also caused by the re°ection of electrons by the nanocavity. The detection of the Moire pattern is tip-dependent. Additionally, an outlook on Spin-Polarized Scanning Tunneling Microscopy (SP-STM) on both surface and subsurface magnetic nanostructures is presented. Possibilities for future research on magnetic nanoislands deposited on metallic substrates are discussed, and the successful growth of these systems is demonstrated in line with reports in literature. Similar to noble-gas-¯lled nanocavities, subsurface metallic nanoislands are able to re°ect electrons that are injected by a (nonmagnetic) STM tip. Preliminary results are presented that show the interference of electrons that could be caused by re°ection of electrons by subsurface metallic nanoislands.