Coronal mass ejections (CMEs) are a key aspect of coronal and interplanetary dynamics. They can inject large amounts of mass and magnetic fields into the heliosphere, causing major geomagnetic storms and interplanetary shocks, a key source of solar energetic particles. Studies over the past decade using the excellent data sets from the SOHO, TRACE, Wind, ACE and other spacecraft and ground-based instruments have improved our knowledge of the origins and early development of CMEs at the Sun and how they affect space weather at Earth. I review some key coronal properties of CMEs, their source regions, their manifestations in the solar wind, and their geoeffectiveness. Halo-like CMEs are of special interest for space weather because they suggest the launch of a geoeffective disturbance toward Earth. However, their correspondence to geomagnetic storms varies over the solar cycle. Although CMEs are involved with the largest storms at all phases of the cycle, recurrent features such as interaction regions and high speed wind streams can also be geoeffective. A new heliospheric experiment, the Solar Mass Ejection Imager, has completed its first year in orbit and I give some early results.
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