The paper first discusses the way in which theories concerning the motion of cold-cathode arcs when acted upon by a magnetic field have ranged from proposing cathode-fall mechanisms for retrograde motion to column processes for forward motion. A very recent mechanism proposed by Ecker for retrograde motion is then examined and found to be capable of extension to higher pressures where forward motion occurs.Previously published experimental results for forward motion are viewed in the light of this mechanism, which is found to be consistent with them, and some features which had hitherto been anomalous may now be explained. Thus there emerges a unified model for the cathode spot and fall region which appears to be valid for both forward and retrograde movements of the arc. It is suggested that this provides a vital step in the approach to the development of certain devices in which arc discharges take place.