The chapter focuses on the welding of thermoplastic polymers. The use of thermoplastics has increased tremendously in the manufacturing industries due to their light-weight characteristic. A detailed study regarding the polymers has been presented and the importance of thermoplastics has also been outlined. The joining technique which has been used in the present work is Friction Stir Welding (FSW). FSW has been one of the major achievements in the field of current welding technologies. Since its invention, the process has been under tremendous research and has been employed to join different metallic alloys of aluminium, magnesium, copper, titanium, etc. The process has also been used to join materials in different joint configurations. Recently, it has been used to weld the thermoplastic materials. An introduction to the FSW technique, the working elements of the process and its constituents have been presented in the chapter. Before the discussion of application of FSW to thermoplastic joining, the other available methods to join thermoplastics such as adhesive bonding and mechanical fastening have been discussed. The literature available with respect to the joining of thermoplastics using FSW has been discussed followed by an experimental study on high density polyethylene (HDPE) sheets. The results of the study have been presented and the relevant conclusions have been drawn.