  Diffraction is an old subject which has received much interest in recent
years due to the advent of diffractive hard scattering. We discuss some
theoretical models and experimental results that have shown new striking
effects, e.g. rapidity gaps in jet and W production and in deep inelastic
scattering. Many aspects can be described through the exchange of a pomeron
with a parton content, but the pomeron concept is nevertheless problematic. New
ideas, e.g. based on soft colour interactions, have been introduced to resolve
these problems and provide a unified description of diffractive and
non-diffractive events. This is part of the general unsolved problem of
non-perturbative QCD and confinement.
