Acceleration of charged particles is an essential ingredient of both magnetospheric substorms and geospace magnetic storms, and is a critical part of space weather. In the case of geospace storms, the ultimate result of particle acceleration is a bulk flow of electric charge through the inner magnetosphere, commonly known as the ring current. This paper is a critical review of the long-standing issue of the storm-substorm relationship, or, in other words, the capability or the necessity of substorms in the build-up of a storm-time ring current. We mainly address the physical effect itself, (e.g., the bulk acceleration of particles), and not the diagnostic of the process, (e.g., the Dst index), which has been the case very often. Within the framework of particle acceleration, substorms retain their storm-importance due to the acceleration potential of substorm-induced impulsive electric fields.
