The stellar magnetic field is a prime ingredient in the interactions between a parent star and its planets. Impacts on the stellar surface or dynamo as well as on the planetary atmosphere and internal structure are expected from these interactions. The magnetic field also plays a huge role in the formation and evolution of the system. The magnetic properties of planet-host stars, however, are barely known. Although it is impossible to spatially resolve the stellar surface of any star other than the Sun, spectropolarimetry allows probing the global large-scale magnetic strength and orientation at a given time. Then, monitoring polarized signatures over time as the star rotates gives us the possibility to reconstruct the topology of the magnetic field at the stellar surface. The method, the planet-host star sample observed so far, and the conclusions obtained from such observations are presented. Fifteen stars with planets have a detected and characterized magnetic field, including the Sun. Although global properties of stars with planets apparently resemble those of stars without known planets, detailed characterization of specific systems has opened a way to probe the energetic environment of exoplanets, with applications on radio emission, habitability, stellar wind/planetary atmosphere interactions, orbital decay, and Ohmic dissipation.