Friction stir welding has gained increasing interest since its invention at TWI some 11 years ago. The continuing development of the process is described. Particular attention is paid to tool features, tool motions and process variants. The design of the tool is the key to the successful application of the FSW process and its ability to accommodate a greater range of materials over a wider range of thickness. A number of different tool designs have been investigated and these are discussed with respect to the materials being welded and the joint geometry. A key factor is the ratio between the swept volume of the probe of the tool and the volume of the probe itself.