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Laser shock processing (LSP) is a new and promising surface treatment technique for improving the fatigue durability, corrosion, wear resistance and other mechanical properties of metals and alloys. During LSP, the generated shock wave can introduce a deep compressive residual stress into the material, due to its high-pressure (GPa-TPa), ultra-fast (several tens nanoseconds), ultra-high strain-rate and high-energy. The overall properties and behavior of metal materials subjected to LSP were significantly improved because a refined surface layer was successfully obtained. Nevertheless, up to now, a clear scenery between micro-structure and macro-property of the refined surface layer, especially formation of sub-micrometer grains from coarse grains during severe plastic deformation, is still pending. Therefore, the basic studies of the underlying mechanism for grain refinement by ultra-high strain-rate presented in this book becomes more and more crucial.