  We review the application of perturbative QCD to diffractive processes. We
introduce the two gluon exchange model to describe diffractive qq(bar) and
qq(bar)g production in deep inelastic scattering. We study the triple Regge
limit and briefly consider multiple gluon exchange. We discuss diffractive
vector meson production at HERA both at t = 0 and large |t|. We demonstrate the
non-factorization of diffractive processes at hadron colliders.
