The basic theoretical and experimental concepts required for an understanding of inelastic electron tunneling spectroscopy (IETS) are briefly reviewed. The emphasis here is on applications of IETS to forbidden transitions in inorganic compounds, although spectra of a few ligands and counterions are also discussed. The similarities between IETS, Raman, and IR spectroscopy will be explored in order to clarify their differences. It is demonstrated that the electron scattering selection rules are weak, thereby allowing the observation of transitions that are totally forbidden to photon processes. Examples of optically forbidden transitions observed as strong bands in IETS are presented. It is also demonstrated that in the IETS spin and orbitally forbidden electronic transitions are often as strong, or stronger, than their optically allowed counterparts.