Introduction to Diffraction and low x Dynamics

An attempt is made to illustrate the relation between low x process es and diffraction. ep scattering provides a unique laboratory, a single hadronic target probed by a point like lepton, where one can try to understand diffraction in terms of a colourless exchange in QCD. Low x processes eventually involve aspects of QCD which cannot be described perturbatively. The HERA inclusive measurements are examined in this respect and compared to results of the Tevatron.