Based on optical studies of pre-breakdown events in negative point-plane gaps, one may distinguish three stages in the liquid breakdown process: (i) the creation of a rapidly expanding vapor cavity adjacent to the point electrode; (ii) the growth of a wave-like disturbance on the surface of the cavity, which we interpret as an electrohydrodynamic instability; and (iii) the runaway growth of the instability leading to streamers that bridge the gap and cause the actual electrical breakdown. The process is probably initiated by current injection from the point cathode, which leads to high local temperature in the liquid adjacent to the electrode. This causes the liquid to vaporize and form the cavity at the point electrode. The electrostatic force on the vapor cavity causes the bubble to expand and leads to interfacial instabilities that propagate as streamers. The subsequent growth of the streamers across the gap has been analyzed by balancing the electrostatic force on the streamer tip with the hydrodynamic drag.