  We review a broad range of phenomena in diffraction in the context of
hadron-hadron, hadron-nucleus collisions and deep inelastic
lepton-proton/nucleus scattering focusing on the interplay between the
perturbative QCD and non-perturbative models. We discuss inclusive diffraction
in DIS, phenomenology of dipole models, resummation and parton saturation at
low $x$, hard diffractive production of vector mesons, inelastic diffraction in
hadron-hadron scattering, formalism of color fluctuations, inclusive coherent
and incoherent diffraction as well as soft and hard diffraction phenomena in
hadron-hadron/nucleus and photon-nucleus collisions. For each topic we review
key results from the past and present experiments including HERA and the LHC.
Finally, we identify the remaining open questions, which could be addressed in
the continuing experiments, in particular in photon-induced reactions at the
LHC and the future Electron-Ion Collider (EIC) in the US, Large Hadron electron
Collider (LHeC) and Future Circular Collider (FCC) at CERN.
