Two-photon physics

A brief discussion is given on the status of two-photon exchange contributions to elastic electron-nucleon scattering. 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. It is also discussed how the imaginary part of the two-photon exchange amplitude can be accessed from the beam normal spin asymmetry in elastic electron-nucleon scattering.