This paper is based upon a project 'Qualistir'(TM) for the on-line quality control of friction stir welds (FSW) in aluminium((I)). Qualistir is a collaborative project lead by RID-Tech with co-industrial partners Vermont, Isotest, Neos Robotics and Gatwick Fusion, and research partners TWI, GKSS and TUS. The objective of the project is to develop an on.-line method for determining the quality of FSW in a butt-welded configuration. The inspection technique described in this paper detects conventional defects and supplies a method for determining whether the weld has been correctly forged which, in turn, safeguards against joint line remnant defects.