The current interest in ever larger solid metal mirrors has highlighted several fabrication problems, amongst them the method of joining the largest available forgings. Thick section fusion welding is still predominantly carried out by multi-pass arc welding processes, but substantial development of electron beam welding (EBW) technology, particularly over the last decade, now warrants re-appraisal of this situation. The electron beam welding process offers not only a single pass welding capability for practically any thickness, but also increased joining rates, reduced distortion and in most cases elimination of consumables. Increases in beam power level have necessitated careful design of both the electron source and electron optics; in addition, to avoid problems of weld defects, special power supply developments have been required. Metal vapor and weld spatter are prevented from entering the electron gun by the introduction of a magnetic trap device and a low stored energy switch mode power source is employed to provide continuous operation, even for the most volatile workpiece materials. Reference is made to the latest in- vacuum external and in-chamber 100 kW gun column developments and, amongst the range of industrial applications, to the particular problems associated with welding large aluminum alloy components.