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This paper reviews vacuum-arc phenomena, and the effect of low-pressure gaseous ambients on electrode phenomena in the transition to atmospheric pressure arcs. The 5 main areas addressed are cathode-spot phenomena, anode-spot phenomena, the properties of the interelectrode plasma for both diffuse arcs and columnar arcs, the interaction of vacuum arcs with axial and transverse magnetic fields, and finally, the transition to atmospheric pressure arcs. The current levels range from 50 A to 50 kA. For each of these 5 main areas, features of the vacuum arc which can be reasonably established from the literature are filrst described, followed by a discussion of parameters requiring additional experimental and theoretical study. For example, the current densities, microstructure, and theoretical description of the cathode spot remain the subject of much debate. There is also a need for additional experimental observations of the anode ion flux and ion energies in order to clarify the overall anode-spot mechanism. With respect to high-current columnar arcs, here there is uncertainty concerning the pressure in the arc column and the mechanism of the grossly evaporating cathode spot. It is firmly established that these high-current columnar arcs can be avoided by applying an axial magnetic field parallel to the arcing axis, but a detailed understanding of the magnetic field/arc interaction remains to be established. The review concludes with a discussion of experimental investigations of electrode phenomena in the presence of low ambient gas pressures.