  After recalling the arguments for possible excess of two-photon contribution
over $\alpha$-counting, model independent statements about the consequences on
the observables will be given. The relevant experimental data are discussed:
(polarized and unpolarized) electron and positron elastic scattering on the
proton, as well as annihilation data. A reanalysis of unpolarized
electron-proton elastic scattering data is presented in terms of the electric
to magnetic form factor squared ratio. This observable is in principle more
robust against experimental correlations and global normalizations. The present
analysis shows indeed that it is a useful quantity that contains reliable and
coherent information. The comparison with the ratio extracted from the
measurement of the longitudinal to transverse polarization of the recoil proton
in polarized electron-proton scattering shows that the results are compatible.
These results bring a decisive piece of information in the controversy on the
deviation of the proton form factors from the dipole dependence.
