Abstract
Most stars have a magnetic field. If the star is cold, this magnetic field is dynamo, i.e. continuously generated by the star itself. If the star is hot, the field is of fossil origin, i.e. a relic of the star's formation. The presence of a magnetic field has far-reaching effects on the star itself, its structure and evolution, its immediate environment and the planets around it. Spectropolarimetry enables us to measure and map magnetic fields on the surface of stars, to better understand them.
