Microquasars are stellar–mass black holes with relativistic jets that mimic, on a much smaller scale, many of the phenomena seen in quasars. Because of their proximity, their study opens the way for a better understanding of the relativistic jets seen elsewhere in the Universe. In microquasars the jets are accelerated and collimated by magnetic activity rather than radiation pressure, and the structure and evolution of the ejecta is determined by magnetic fields. In clouds expelled during quasi–periodic oscillations of tens of minutes in GRS 1915+105, equipartition magnetic fields have values of ca. 10 G. In ejecta with bulk motion speeds greater than or equal to 0.9c, linear polarizations of up to 10% have been measured, and as in quasars, far from the central engine the magnetic field is well ordered and aligned at right angles to the jet axis.