In our search for lighter structures using less energy and materials, methods of joining can have a significant impact. Friction welding is one of the oldest joining processes for metallic materials. Rotary friction welding was invented in the 19th Century but its application was limited to relatively small components. However, in 1991, engineers at TWI in Cambridge invented another variant of friction welding called friction stir welding (FSW). This fundamental development has completely transformed some branches of manufacturing, particularly those making large aluminium fabrications. Now the most modern aluminium railway vehicles, high speed ships and cars are manufactured using this technology. The process is also finding new applications in the aerospace sector, where riveting has always been preferred over welding for airframes. Recently FSW has been a key enabling technology for reducing construction and operating costs for airframes to the extent that the air taxi is about to become a reality.