We analyze the physical phenomena occurring during the breakdown of gas gaps within times on the order of several nanoseconds or less. It is noted that the streamer mechanism has low probability if the times of formation of the discharge and the fall-off of the voltage across the gap are commensurate with the emission times of the excited molecules. The development of a Townsend discharge is made difficult by the strong influence of the space charge of the avalanche. The discharge mechanisms in single-electron and multi-electron initiation are described and the features of their development are outlined. Oscillographic and electron-optical measurement procedures are described and the main experimental results are reported. Theories explaining the main regularities are presented together with the mechanism for initiation and development of a discharge in ultrastrong electric fields. It is noted that x-radiation is produced in the discharge. Results are reported on a discharge at a voltage below the static breakdown value, initiated by fast electrons and by intense ultraviolet radiation.