Electrode erosion in high current, high energy spark gaps was examined both theoretically and experimentally. A brief description is given of the production of high velocity, high temperature electrode vapor jets which are shown experimentally to be the major source of electrode erosion under these conditions. Consideration of a simple one dimensional thermal model with the vapor jet considered as a transient heat flux onto the electrode surface led to a figure of merit for the electrode materials which was in close agreement with experiments. The rapid decrease in erosion with increasing gap separation was also measured and explained by the vapor jet mechanism. Erosion rates are given for a large number of electrode materials including many copper composites, such as copper niobium and copper tungsten. Possible implications of these results on rail gun design are also given.