  Diffractive reactions involving a hard scale can be understood in terms of
quarks and gluons. These reactions have become a valuable tool for
investigating the low-x structure of the proton and the behavior of QCD in the
high-density regime, and they may provide a clean environment to study or even
discover the Higgs boson at the LHC. In this paper we give a brief introduction
to the description of diffraction in QCD. We focus on key features studied in
ep collisions at HERA and outline challenges for understanding diffractive
interactions at the LHC.
