  The presence of two photon exchange in $ep$ elastic scattering and in the
crossed processes ($\bar p +p\leftrightarrow \ell^+ +\ell^-$, $\ell=e$ or
$\mu$) is discussed in terms of three complex amplitudes which are functions of
two kinematical variables, the momentum transfer squared $q^2$ and the
polarization of the virtual photon, $\epsilon$. Model independent expressions
of the differential cross section and of polarization observables are derived.
Particular attention is devoted to the extraction of hadron electromagnetic
form factors which becomes much more difficult in presence of two photon
exchange, and to the relevant experimental observables which could give
evidence for such mechanism.
