Theories and experimental facts of vacuum breakdown are reviewed. Measurements with gap spacings of 0.015 inch indicate little or no variation of breakdown with tube pressure between approximately 10-5and 10-7torr and with frequency of the applied voltage from 0 to 6 Mc/s. Variation of electrode materials, geometry, and surface preparations provide no marked improvements, but high voltage aging or conditioning does improve the breakdown characteristics by a factor of as much as 2 to 1. Enhanced field emission, from whisker-like protrusions on the cathode surface, causes individual beams of field-emission prebreakdown current to flow from cathode to anode. This prebreakdown current can cause resistive heating of the protrusion and localized electron-bombardment anode heating both of which produce vaporization and destructive effects which could lead to breakdown.