The mechanism of inverse photoemission is explored by comparing inverse photoemission data from free-electron-like materials, such as Al, with first principles calculations of the cross section. Thereby, one can identify the inverse of the surface photoelectric effect and the decay of bulk plasmons into light. Plasmon decay can strongly affect inverse photoemission from thin film systems and lead to striking resonance phenomena. The application of inverse photoemission to semiconductors yields two- and three-dimensional band dispersions of surface states and conduction bands. It turns out that inverse photoemission measurements provide a stringent test of the state-of-the-art quasiparticle calculations, because the self-energy correction to the conventional (local density) bands is much larger for unoccupied than for occupied states. Examples include Si, Ge, III-V's, dangling bonds and ordered overlayers. Future directions for inverse photoemission are also discussed, including the measurement of holes that are responsible for superconductivity in high Tc superconductors. © 1990 IOP Publishing Ltd.