The magnetospheric substorm is perhaps the most widely known yet most poorly understood facet of magnetospheric disturbances. It is generally agreed that the substorm results from an explosive conversion of magnetic energy generated by the interaction between the solar wind and the magnetosphere. The energy thus released causes a variety of magnetospheric and polar upper atmospheric disturbances. Although much of the interpretation of substorm observations is still a matter of great controversy, we have made considerable progress in understanding substorm phenomena in terms of plasma processes, most of which are fundamental in cosmic electrodynamics.