Photoemission and inverse photoemission (or bremsstrahlung spectroscopy) are shown to be complementary techniques for probing occupied and unoccupied electronic states, respectively. The momentum of delocalized valence states can be measured as well as their energy if low electron (photon) energies in the 10–40 eV range are used. Thus, energy band dispersions are obtained for bulk, surface, and adsorbate states which cannot be determined by other techniques. A status report of inverse photoemission is given and illustrated by results for bulk states in ferromagnets and silicides, for broken bond states on silicon surfaces, and for unoccupied molecular adsorbate orbitals.