Theoretical review of diffractive phenomena

We review QCD based descriptions of diffractive deep inelastic scattering emphasising the role of models with parton saturation. These models provide natural explanation of such experimentally observed facts as the constant ratio of the diffractive and total cross sections as a function of the Bjorken variable, and Regge factorization of diffractive parton distributions. The Ingelman-Schlein model and the soft color interaction model are also presented.