  Two-photon exchange contributions to elastic electron-scattering are
reviewed. The apparent discrepancy in the extraction of elastic nucleon form
factors between unpolarized Rosenbluth and polarization transfer experiments is
discussed, as well as the understanding of this puzzle in terms of two-photon
exchange corrections. Calculations of such corrections both within partonic and
hadronic frameworks are reviewed. In view of recent spin-dependent electron
scattering data, the relation of the two-photon exchange process to the
hyperfine splitting in hydrogen is critically examined. The imaginary part of
the two-photon exchange amplitude as can be accessed from the beam normal spin
asymmetry in elastic electron-nucleon scattering is reviewed. Further
extensions and open issues in this field are outlined.
