This monograph gives a comprehensive and up-to-date overview of hard and soft diffraction processes in strong interaction particle physics. The first part covers soft hadron--hadron scattering in a complete and mature presentation. It can be used as a textbook in particle physics classes. Chapters 8-11 address graduate students as well as researchers, covering hard diffraction and, in particular, deep inelastic scattering, and present very new results and experimental findings and their phenomenological interpretations. This should also attract readers involved in modern research work. The first part contains the optical analogy, the eikonal approximation of quantum mechanics, high-energy kinematics. The second is a review of "old diffraction" (Regge theory etc.), while the third part is devoted to the "new diffraction", the pomeron in QCD.