Inelastic Electron Tunneling Spectroscopy (IETS) is a method to measure vibronic and electronic excitation of molecules deposited as interlayers in tunnel junctions. In this article the method itself is briefly, discussed and compared with other techniques of, surface vibrational spectroscopy. As examples, investigations by IETS on sublimated phthalocyanine dye molecules, on ultrahigh vacuum prepared Al/Al oxide/Pb tunnel junctions and on protonirradiated Al/Al oxide+HCOO−/Pb tunnel junctions are reviewed. Tunneling measurements performed on n-Si/SiO2/metal structures with very thin SiO2 layers are reported and the results are discussed.