Scroll shoulder tool is a relatively new friction stir welding (FSW) tool introduced by The Welding Institute of UK in 1999. It offers extra advantages of eliminating the tilted tool axis that is usually used in conventional tool FSW and performing non-linear thick section FSW with a relatively simple machine. However, the forming mechanism of its shoulder flow zone still remains unidentified. This mechanism is fundamental in designing a proper scroll shoulder tool to produce a defect-free weld. This monograph, firstly, systematically reviewed FSW tool design, weld zone material flow pattern and flow forming mechanism, and scroll shoulder tool shoulder flow zone formation to date. Then, a novel scroll shoulder tool design and experimental techniques were detailed. Subsequently, the experimental results were presented. Finally, the shoulder flow zone forming mechanism and strategy to secure weld quality for scroll shoulder tool were suggested. This study has revealed for the first time the forming mechanism of shoulder flow zone and approaches to control weld quality for scroll shoulder tool. This monograph is an excellent reference for FSW researchers, engineers and practitioners.