The most dramatic changes in the solar corona occur during coronal mass ejections, or CMEs, during which solar material is propelled outward into the heliosphere from regions in the corona not previously participating in the solar wind expansion. These ejections often produce major solar wind disturbances, large geomagnetic storms, enhancements of moderate energy (MeV) particle fluxes and decreases in the galactic cosmic ray particle flux. This paper provides an overview of the CME phenomenon with emphasis on the CMEs themselves and how they evolve as they propagate out through the heliosphere.