[en] In (e,2e) or electron coincidence spectroscopy, kinematically complete measurements are made of symmetric ionizing collisions initiated by an electron beam on a gaseous atomic or molecular target. The reaction measures the separation (or binding) energies of electrons in the valence shell an the shapes in momentum space of individual orbitals. It is sensitive to the low to medium momentum region and therefore provides a sensitive measurement of the charge density far from the atomic nuclei. This is the chemically interesting region. It also provides quantitative information on electron correlations in both the initial target and final ion many body states