This book introduces the principles and characteristics of friction stir welding and processing. Based on the inherent issues of friction stir welding, such as back support, weld thinning, and keyhole defects, the book summarizes innovative technologies related to solution strategies and presents a wide range of examples. It introduces the advantages and joining mechanism of friction stir welding in the joining of dissimilar materials and explains the importance of combining metallurgical bonding and mechanical joining. It also includes the characteristics of friction stir processing in terms of microstructure refinement, mechanical properties, surface modification, and the preparation of composites. This book is of interest to a broad readership in various fields of materials science and engineering.