The friction stir welding (FSW) process was invented in 1991 by Wayne Thomas et al., one of the authors of this chapter. This machine tool based process is currently considered an important development in welding technology, saving costs and weight for a steadily expanding range of applications of lightweight metallic structures. Evidences of the disruptive character of the FSW process are the prompt adoption by world-wide industry of the significant advantages of FSW and the numerous technic-scientific papers and patents published. The FSW technology has been subjected to the most demanding quality standard requirements and used in challenging industrial applications over a wide range of structural and non-structural components. In this chapter, some of the basic fundamentals underpinning the invention of FSW technology are presented with emphasis for the concept of the third-body region. The state-of-the-art concerning tooling in FSW for conventional and bobbin stir welding approaches are introduced. The non-destructive testing assessment of the most relevant imperfections in FSW is also discussed for butt and lap joints. In summary, the FSW is a key joining technology for lightweight metallic structures. The international organization for standardization standard for welding aluminium alloys by FSW is available and the most recent European standards for design of structures—Eurocodes, already include guidelines for the application FSW process.