Existing theories for the initiation of electrical breakdown are reviewed, together with the experimental observations on which they are based. Experiments here described have extended the available data on electrical breakdown between broad area electrodes under ultra-high-vacuum conditions. The results, together with those of several other experimenters, are interpreted on the basis of a single picture which explains and relates the phenomena of predischarge currents and the initiation of breakdown. Based on field emission from sharp submicroscopic points, this picture predicts breakdown when the local electric field at the cathode reaches a critical value. The local field, which for broad area electrodes may be much larger than the average field, is deduced from observations of field emission prior to breakdown. When properly analyzed, data for tungsten electrodes from this research and several others indicate a value for the electrical field at breakdown which is independent of gap spacing or geometrical configuration for voltages up to 250 kV. The critical breakdown field for tungsten is 6.5×107 V∕cm. The above picture also gives physical insight into other phenomena associated with electrical breakdown.