Virtual compton scattering

The talk will review the potential interest of virtual Compton scattering (VCS) for understanding of nucleon structure. The important experimental aspects of the reaction will first be recalled. The electroproduction of photons is the coherent sum of the VCS process, where the photon is produced on the proton, and of the Bethe-Heitler (BH) process where the photon is emmitted by the electron. Since the BH amplitude is calculable from QED, the interference between the VCS and the BH amplitude is a priori interesting but it will be pointed out that this must be taken with a grain of salt. The talk will concentrate on the two most promising kinematical regions. First the threshold region which allows to measure the 10 generalized polarizabilities of the proton. Here the VCS-BH interference is entirely under control. Some theoretical an preliminary experimental results will be discussed. Next is the hard scattering regime where perturbative QCD can apply. Contrary to the threshold case, the amplitude has a phase which is a non trivial prediction of PQCD. It will be explained how out of plane electron spin asymetries can be used to detect this phase. The results of a cliquark calculation, appropriate to a 10 GeV upgrade of CEBAF will be presented. The pure QCD calculations which could be tested at ELFE, will be discussed qualitatively