Coronal mass ejections (CMEs) are spectacular ejections of material from the Sun as seen in the coronal field of view. Regular observations are possible with both ground-based and space-based coronagraphs. I present our current understanding of CMEs based on multi-wavelength observations from groundbased instruments as well as from space missions such as SoHO. Based on the continuous and multi-wavelength observations of CMEs from SoHO over a period of more than a solar cycle, the physical properties of CMEs are described. Recent observations of CMEs with the SECCHI coronagraphs, namely COR1 and COR2 aboard the twin STEREO spacecrafts A and B are also presented. STEREO surpasses previous missions by providing a 3-D view of CME structure from two vantage points. Applications of STEREO observations to 3-D reconstructions of the leading edge of CMEs are described.