This paper first presents an experimental electrical and optical study of the development of an electrical discharge in water. The point–plane water gap is subjected to a 0.02 µs/350 µs impulse voltage. A Schlieren device associated with an image converter or a photomultiplier demonstrates that the discharge phenomenon requires heating of the water located around the extremity of the point. This thermal process leads to the formation of gas bubbles in which an electrical discharge propagates. In the experimental conditions a threshold value of 80 J is necessary to create bubbles. No UV or IR light emission is recorded before the presence of bubbles is detected. When the energy conditions are sufficient (⩾200 J), the volume of bubbles grows until the whole inter-electrode space is filled; then a breakdown of the gap occurs. When this happens, a high amplitude pressure shock wave is generated. In the second phase of this work the shock wave created by the gap breakdown was studied for energy levels up to 100 kJ. It is clearly pointed out that the pressure shock wave peak value depends on the energy remaining at breakdown time. For a constant remaining energy, the peak pressure value increases with increasing gap length.