Coronal mass ejections (CMEs) are the most energetic events in the solar system, expelling up to 1016 g of coronal material at speeds of several hundreds or thousands of km s−1 from the Sun. As CMEs are the primary cause of space weather disturbances, we need to understand their underlying cause(s) in order to be able to predict them. After an overview of their basic properties based on multi-wavelength and multi-instrument data, including optical, EUV, X-ray and radio observations from microwaves to kilometric wavelengths, we follow CMEs from the low solar atmosphere through the interplanetary medium to the Earth. A discussion on CME source regions is presented, followed by a discussion on theoretical CME models, comparing them to observations. Evidence is emerging that magnetic helicity is the key to understand CMEs: they go off when too much helicity has built up in the corona. Therefore, in the second part an overview of this young and dynamic field of solar physics is presented. During the last four years, attempts were made to estimate/measure magnetic helicity from solar and interplanetary observations. As magnetic helicity (unlike current helicity) is one of the few global quantities that is conserved even in resistive MHD on a timescale less than the global di usion timescale, magnetic helicity studies make it possible to trace helicity as it emerges from the sub-photospheric layers into the corona, then being ejected via CMEs into the interplanetary space, and reaching the Earth in a magnetic cloud. Observational studies on the relative importance of di erent sources of magnetic helicity investigate whether the dominant helicity source is photospheric plasma motions (photospheric di erential rotation and localized shearing motions) or the twist of the emerging flux tubes created under the photosphere (presumably by the radial shear in the di erential rotation in the tachocline).