Abstract
The first stages of star formation are accompanied by powerful and spectacular matter ejections, whose origin is still a mystery, but which appear to be an essential ingredient in the star formation process. I'll present their main characteristics, as well as the unprecedented details of their structure recently revealed by large telescopes in space (HST, JWST) and on the ground (ALMA, NOEMA). I'll then discuss the different scenarios envisaged for their origin, their impact on stellar and planetary formation, and the latest avenues of research in this field.
