Electron momentum spectroscopy of atoms, molecules, and solids is based on (e,2e) reactions that observe the distribution of recoil momenta for energy-resolved states of the residual system. It is interpreted simply in terms of the momentum-space orbitals of the independent-particle model. The relevant ideas originated in nuclear physics. The earliest experiments observed that strongly excited final states belong to orbital manifolds that extend the independent-particle ideas to correlated systems. Some weakly excited final states do not belong to orbital manifolds. They give sensitive information about target ground-state correlations. The energy-momentum distribution of valence bands is observed for solids. Calculations for atoms, molecules, and crystals converge to the experimental result as the structure calculation is improved.
