Two Photon Physics in Elastic Electron-Nucleon Scattering

I discuss the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. The process on the nucleon is related to the generalized parton distributions which also enter in other wide angle scattering processes. It is found that the two-photon exchange contribution is able to resolve to a large extent the difference between electric form factor measurements from Rosenbluth and polarization transfer experiments