  XEUS is a large area telescope aiming to rise X-ray Astronomy to the level of
Optical Astronomy in terms of collecting areas. It will be based on two
satellites, locked on a formation flight, one with the optics, one with the
focal plane. The present design of the focal plane foresees, as an auxiliary
instrument, the inclusion of a Polarimeter based on a Micropattern Chamber. We
show how such a device is capable to solve open problems on many classes of
High Energy Astrophysics objects and to use X-ray sources as a laboratory for a
substantial progress on Fundamental Physics.
