  Coherence phenomena, the increase with energy of coherence length 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 $M^2\ll Q^2$ and the color transparency
phenomenon 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 the inconsistency of QCD
predictions for inclusive and exclusive processes and 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.
