I will discuss experiments in which we have used electron tunneling to measure the spectrum of individual “electron-in-a-box” energy levels in nanometer-scale metal particles and quantized states in single molecules. In metal particles, the level spectra do not correspond to free electrons, but rather they are modified by all of the various forces which act on electrons inside metals. I will describe the different consequences of superconducting pairing, magnetic exchange, spin-orbit coupling, and magnetic anisotropy forces. Single-molecule electrical devices display many of the same phenomena seen in larger-scale samples, such at Coulomb blockade and the Kondo effect, and they also exhibit additional features associated with the coupling of electron tunneling to molecular motion and to the spin states of magnetic molecules.
