The existence of the ring current had been predicted since the early twentieth century, but the carrier of the ring current had not been confirmed until 1967. The intensity of the ring current decreases gradually during the recovery phase of a magnetic storm. The decay of the ring current is attributed to loss of the ring current ions. Before the discovery of the carrier of the storm-time ring current, charge exchange with neutral hydrogen was suggested to remove the ions trapped in the magnetic field line. The ions originating in the nightside magnetosphere pass through the inner magnetosphere and contribute to the enhancement of the ring current. The ring current has an impact on the ions constituting the ring current itself. The ring current may have large influences in view of space weather. One of the striking impacts of the ring current would appear in geomagnetically induced current on power grid system.