[en] The properties of the explosive (i.e., cold cathode) emission plasmas formed on the cathodes of intense beam diodes and magnetically insulated transmission lines significantly affect the behavior of these devices. A review of the current understanding of explosive emission processes is given here. This paper emphasizes work reported outside the general pulsed power literature but nonetheless relevant to diodes and MITL's, such as the extensive studies of vacuum arcs and the Soviet studies of single emitters and individual cathode spots. Particular attention is given to the significance of dielectric contaminations on the cathode and to the presence of fast ions emitted from cathode spots. The dielectric inclusions and surface layers which exist on cathodes in typical vacuum systems can have several effects. Microscopic studies of vacuum breakdown have shown that breakdown often occurs at the location of dielectric inclusions; explanations for this range from simple field enhancement at the dielectric-metal triple points to semiconductor physics effects such as band bending or electroformed filaments