Knock in gasoline engines is due to an explosion in the preflame gases. Records of simultaneous light emission, pressure, and ionization signals require this interpretation. Before the occurrence of the knock pressure pulse, the pressure-time curves for the knocking cycles showed the same mean values and variations in pressure and crankangle time as with the nonknocking cycles. The pressure pulse was accompanied by simultaneous emission of light from a window and the triggering of a nearby ionization gap. The pressure pulse appeared to attenuate while traversing the chamber at essentially sonic velocity to a remotely located pressure pickup.