Observations and numerical simulations of the response of the thermosphere and ionosphere to “superstorms” illustrate that multiple processes are operating. The initial response at high latitude is thermospheric heating, thermal expansion, high-velocity winds, wave surges, the initiation of a new global circulation, and the start of neutral composition changes. At low latitude, the initial response is driven by the penetration of magnetospheric electric fields, moving the equatorial ionization anomaly poleward, and enhancing both F region plasma densities and the total electron content at midlatitudes and low latitudes. Electron content also increases dramatically at the higher altitudes. In the later stages of the response, plasma densities begin to respond to the changing circulation; the transport of composition changes to midlatitudes and low latitudes; and the generation of disturbance dynamo effects, which either compete or combine with penetration fields. The observations and modeling indicate that all the processes have a significant impact at some time and place during the storm.