High-energy diffraction has become a hot and fashionable subject in recent years due to the great interest triggered by the HERA and Tevatron data. These data have helped to show the field from a different perspective paving the road to a hopefully more complete understanding than hitherto achieved. The forthcoming data in the next few years from even higher energies (LHC) promise to sustain this interest for a long time. We believe that it is therefore necessary to summarize the main developments which have marked the growth of high-energy diffractive physics in recent decades, and to assess the present state of the art. This is the purpose of the present book, which is especially aimed at the young researchers who are entering the field and want to get acquainted with the relevant results and the main theoretical techniques. The "new" diffraction has started to bridge the gap between the hard and soft regimes of strong interactions. A modern account of the subject, in our opinion, should reflect this situation, covering both the traditional approaches to soft processes which are still alive and useful, and the modern treatment of hard dynamics in the framework of perturbative QCD. The book is divided into three parts. The first part (Chaps. 1-3) contains some introductory material: the systematics of diffractive processes, some historical remarks, the optical analogy, the eikonal approximation of quantum mechanics, and high-energy kinematics. In the second part (Chaps.