Observing electron motion in solids

We describe an experimental technique that allows for the direct determination of the motion (i.e., the momentum) of electrons in atoms, molecules, and solids. This ($e$, $2e$) technique centers on a fast electron's ejecting a second electron from a target. Precise spectroscopy of both electrons' final energies and momenta provides very significant information on the second electron's initial state. Decades of past ($e$, $2e$) studies on single-molecule states have now progressed into studies of condensed matter. The interpretation of these experiments is mediated in this colloquium by analysis of intermediate models in the form of chainlike molecules.