  We review the evolution of the studies of diffractive processes in the strong
interaction over the last 60 years. First, we briefly outline the early
developments of the theory based on analyticity and unitarity of the S-matrix,
including the derivation and exploration of the Regge trajectories and related
moving cuts. Special attention is paid to the concept of the Pomeron trajectory
introduced for description of total, elastic and diffractive cross sections at
high energies and to the emergence of the dynamics of multi-Pomeron
interactions.The role of large longitudinal distances and color coherent
phenomena for the understanding of inelastic diffraction in hadron-hadron
scattering and deep inelastic scattering is emphasized. The connection of these
phenomena to the cancellation of the contribution of the Glauber approximation
in hadron-nucleus collisions and to the understanding of the Gribov-Glauber
approximation is explained. The presence of different scales in perturbative
QCD due to masses of heavy quarks has led to the emergence of numerous new
phenomena including non-universality of the slopes of Regge trajectories made
of light and heavy quarks and non-universal energy dependence of elastic cross
sections. The application of the perturbative QCD techniques allowed us to
calculate from the first principles the interaction of small transverse size
color singlets with hadrons leading to the development of the quantitative
theory of hard exclusive reactions and to the successful prediction of many
regularities in hard large mass diffraction. It also led to the prediction of
the phenomenon of complete transparency of nuclear matter in QCD in special
processes. The conflict of perturbative QCD with probability conservation for
high energy processes of virtual photon-nucleon scattering is explained. Some
properties of the new QCD regime are outlined.
