Because geomagnetic storms can have important effects on communications and electrical power distribution, an ability to predict these storms has considerable value. We have recently come to understand that coronal mass ejections (CMEs) cause most large geomagnetic storms during the most active part of the solar cycle [Gosling et al., 1991]. CMEs are huge eruptions of hydrogen gas that originate low in the corona; they are often associated with eruptive prominences and solar flares. CMEs can travel as much as three or four times faster than the slower solar wind plasma ahead of them. As a fast CME travels outward through the solar wind toward Earth, both the material ahead of the CME and the leading portion of the CME itself are compressed, shocks are formed, and magnetic field lines from the ambient solar wind are draped across the front of the CME. When this disturbance reaches the Earth, the Earth's magnetic field is affected.