Spectroscopic measurements with the scanning tunneling microscope (STM) for metal overlayers on the GaAs(110) surface are reviewed, including results for submonolayer Sb, Au, and Fe adsorbate systems. It is found that a characteristic spectrum of band gap states is present in each of these systems, induced by certain disorder-related structures on the surface. The observation of these gap states provides a phenomenological description of Fermi-level pinning on this surface. An extensive discussion is presented on the physical origin of the spectrum of gap states. The character of the states is considered, including possible contributions from evanescent (complex wavevector) semiconductor states, metal/semiconductor bonds, and surface Ga dangling bonds. The connection between the experimental observations and recent theoretical works is also discussed. © 1992.