Coronal mass ejections (CMEs) are eruptions into interplanetary space of as much as a few billion tons of plasma and embedded magnetic fields from the Sun's corona. In contrast to the steady-state solar wind, CMEs originate in regions where the magnetic field is closed and result from the catastrophic disruption of large-scale coronal magnetic structures, such as coronal streamers. The exact processes involved in the release of CMEs are not known. CMEs can occur at any time during the solar cycle, but their occurrence rate increases with increasing solar activity and peaks around solar maximum. CMEs are propelled outward at speeds ranging from <50 to ~2000 kilometers per second. Fast CMEs --those traveling faster than the ambient solar wind-- are responsible for triggering large, nonrecurrent geomagnetic storms when they encounter the Earth's magnetosphere. Such storms can result from the passage either of the CME itself or of the shock created by the fast CME's interaction with the slower-moving solar wind. The majority of large and major geomagnetic storms are generated by the encounter with both the interplanetary shock and the CME that drives it. The "geoeffectiveness" of CMEs --i. e., their ability to disturb the Earth's magnetosphere-- is a function of their speed, the strength of their magnetic field, and the presence of a strong southward magnetic field component. The image above shows a CME that occurred on January 15, 1996. The bright ring inside the dark disk indicates the location and diameter of the Sun. The outer edge of the disk lies at ~3.5 solar radii. This white-light image was acquired with the C3 coronagraph developed by the Naval Research Laboratory as part of the Large-scale and spectrometic coronagraph (LASCO) experiment on SOHO. (Courtesy of the SOHO/LASCO consortium. SOHO is a project of internatinal collaboration between ESA and NASA. This figure, along with many other CME images, can be found on the Web at the SOHO Gallery.)