  Multiple interaction models satisfying $s$-channel unitarity predict that, in
contrast to inelastic processes, factorization is violated in diffractive
processes. The size of this effect can be characterized in terms of the
rapidity gap survival probability. The possibility of its measurement at HERA
is pointed out. Furthermore a method to measure photon diffraction dissociation
at LEP2 and planned linear colliders is discussed and cross section predictions
are given.
