Deeply exclusive processes and generalized parton distributions

We discuss how generalized parton distributions (GPDs) enter into hard exclusive processes, and focuss on the links between GPDs and elastic nucleon form factors. These links, in the form of sum rules, represent powerful constraints on parametrizations of GPDs. A Regge parametrization for the GPDs at small momentum transfer –t is extended to the large-t region and it is found to catch the basic features of proton and neutron electromagnetic form factor data. This parametrization allows to estimate the quark contribution to the nucleon spin. It is furthermore discussed how these GPDs at large-t enter into two-photon exchange processes and resolve the discrepancy between Rosenbluth and polarization experiments of elastic electron nucleon scattering.