Friction stir welding (FSW) is a new welding process developed at The Welding Institute in Cambridge, U.K. This process uses a non-consumable rotating third body to generate frictional heat and create forging to facilitate continuous solid-state joints. In this paper, the current state of the art of FSW is discussed. A preliminary description of the process is provided, followed by the results of some relatively simple thermal modeling. The modeling results are used to provide a description of temperature distributions in FSW, as well as illustrate the effects of variations in process conditions. Representative microstructures of FSW on an Al 6061 alloy are then presented. Properties of these friction stir welds are then discussed and compared to those of both the base metal and to comparable GTAW welds. Some discussion is then given to the effects of section thickness on FSW. Examples are given of friction stir welds on aluminum alloys ranging from 2 to 30 mm in thickness. A brief description is then given of efforts to friction stir weld various other materials. Thermal stability of the tool is defined as a critical characteristic of the process. As a result, low melting temperature materials are generally readily friction stir-weldable; however, higher temperature materials generally cause degradation of the tool. Finally, a short discussion of future trends in FSW is given. With regard to aluminum, needs appear to be for greater productivity and flexibility. For other materials, however, development of better tool materials and improved thermal management are the key issues.