Small x physics

The QCD expectations concerning the small x limit of parton distributions, where x is the Bjorken scaling variable, are reviewed. This includes discussion of the evolution equations in the small x region, the Lipatov equation, which sums the leading powers of ln(1/x), and the shadowing effects. Phenomenological implications of the theoretical expectations for the deep inelastic lepton-hadron scattering in the small x region, which will be accessible at the HERA ep collider, are described. We give predictions for structure functions F2 and FL and discuss specific processes sensitive to the small x physics such as heavy quark production, deep inelastic diffraction and jet production in deep inelastic lepton scattering. A brief review of nuclear shadowing in the inelastic lepton nucleus scattering at small x is also presented. (orig.)