  An Electron-Ion Collider (EIC) would enable next-generation measurements of
deep-inelastic scattering (DIS) on the deuteron with detection of a
forward-moving nucleon (p, n) and measurement of its recoil momentum
("spectator tagging"). Such experiments offer full control of the nuclear
configuration during the high-energy process and can be used for precision
studies of the neutron's partonic structure and its spin dependence, nuclear
modifications of partonic structure, and nuclear shadowing at small x. We
review the theoretical description of spectator tagging at EIC energies
(light-front nuclear structure, on-shell extrapolation in the recoil nucleon
momentum, final-state interactions, diffractive effects at small x) and report
about on-going developments.
