Observations from several satellites in geosynchronous orbit, combined with solar wind observations from the ACE spacecraft, demonstrate that cold, dense plasma sheet material can be delivered into the inner magnetosphere under two scenarios: (1) a sudden southward IMF preceded by an extended interval of northward field and (2) a very strong magnetospheric compression caused by a large increase in the solar wind dynamic pressure, also under conditions of northward IMF. Examination of 30 “superdense” plasma sheet events observed during 15 months in 2001 and 2002 shows that these two mechanisms are probably responsible for the occurrence of most superdense events, if “northward IMF” is generalized to “northward or very weak IMF B_z.” Previous work has shown that a superdense plasma sheet contributes to a strong ring current and geomagnetic disturbance if it is accompanied by strong magnetospheric convection (which requires southward IMF). Hence this study suggests that the formation of a strong geomagnetic storm should be favored by an extended interval of strong southward IMF that is preceded by an earlier interval of northward field or very weak B_z, during which the plasma sheet fills with cool, dense plasma.