  Coherence phenomena, and the non-universality of parton structure of the
effective Pomeron are explained. New hard phenomena directly calculable in QCD
such as diffractive electroproduction of states with $M^2\ll Q^2$ as well as
new options to measure the light-cone wave functions of various hadrons are
considered. An analogue of Bjorken scaling is predicted for the diffractive
electroproduction of $\rho$ mesons at large momentum transfers and for the
production of large rapidity gap events, as observed at HERA. A
phenomenological QCD evolution equation is suggested to calculate the basic
characteristics of the large rapidity gap events. The increase of parton
densities at small $x$ as well as new means to disentangle experimentally soft
and hard physics are considered. We discuss constraints on the increase of deep
inelastic amplitudes with $Q^2$ derived from unitarity of the S matrix for
collisions of wave packets. New ways to probe QCD physics of hard processes at
large longitudinal distances and to answer the long standing problems on the
origin of the Pomeron are suggested. Unresolved problems and perspectives of
small $x$ physics are also outlined.
