Double-electrode　gas　metal　arc　welding　(DE-GMAW)　is　a　novel　welding　process　in　which　a　second　electrode,　non-consumable　or　consumable,　is　added　to　bypass　part　of　the　wire　current.　The　bypass　current　reduces　the　heat　input　in　non-consumable　DE-GMAW　or　increases　the　deposition　rate　in　consumable　DE-GMAW.　The　fixed　correlation　of　the　heat　input　with　the　deposition　in　conventional　GMAW　and　its　variants　is　thus　changed　and　becomes　controllable.　At　the　University　of　Kentucky,　DE-GMAW　has　been　tested/developed　by　adding　a　plasma　arc　welding　torch,　a　GTAW　(gas　tungsten　arc　welding)　torch,　a　pair　of　GTAW　torches,　and　a　GMAW　torch.　Steels　and　aluminum　alloys　are　welded　and　the　system　is　powered　by　one　or　multiple　power　supplies　with　appropriate　control　methods.　The　metal　transfer　has　been　studied　at　the　University　of　Kentucky　and　Shandong　University　resulting　in　the　desirable　spray　transfer　be　obtained　with　less　than　100A　base　current　for　1.2　mm　diameter　steel　wire.　At　Lanzhou　University　of　Technology,　pulsed　DE-GMAW　has　been　successfully　developed　to　join　aluminum/magnesium　to　steel.　At　the　Adaptive　Intelligent　Systems　LLC,　DE-GMAW　principle　has　been　applied　to　the　submerged　arc　welding　(SAW)　and　the　embedded　control　systems　needed　for　industrial　applications　have　been　developed.　The　DE-SAW　resulted　in　1/3　reduction　in　heat　input　for　a　shipbuilding　application　and　the　weld　penetration　depth　was　successfully　feedback　controlled.　In　addition,　the　bypass　concept　is　extended　to　the　GTAW　resulting　in　the　arcing-wire　GTAW　which　adds　a　second　arc　established　between　the　tungsten　and　filler　to　the　existing　gas　tungsten　arc.　The　DE-GMAW　is　extended　to　double-electrode　arc　welding　(DE-AW)　where　the　main　electrode　may　not　necessarily　to　be　consumable.　Recently,　the　Beijing　University　of　Technology　systematically　studied　the　metal　transfer　in　the　arcing-wire　GTAW　and　found　that　the　desired　metal　transfer　modes　may　always　be　obtained　from　the　given　wire　feed　speed　by　adjusting　the　wire　current　and　wire　position/orientation　appropriately.　A　variety　of　DE-AW　processes　are　thus　available　to　suit　for　different　applications,　using　existing　arc　welding　equipment.　(C)　2013　The　Society　of　Manufacturing　Engineers.　Published　by　Elsevier　Ltd.　All　rights　reserved.