Polymers are today’s most widely used materials. Accordingly, the joining technologies of these materials are a popular and extensively researched area. Of these technologies, welding procedures are the most important in industry and research. For polymers, one of the most promising welding technologies is friction stir welding (FSW). The technology has many advantages; it can be easily automated and it has a small impact on the environment. Also, the procedure can be used to weld thicker polymer sheets and thermoplastic polymer matrix composites. In this study, we present the technology of friction stir welding and its individual variants (procedure, tool modifications), based on recently published studies, and review current and future research directions.