  Accretion onto compact objects plays a central role in high energy
astrophysics. In these environments, both general relativistic and plasma
effects may have significant impacts upon the propagation of photons. We
present a full general relativistic magnetoionic theory, capable of tracing
rays in the geometric optics approximation through a magnetised plasma in the
vicinity of a compact object. We consider both the cold and warm, ion and pair
plasmas. When plasma effects become large the two plasma eignemodes follow
different ray trajectories resulting in a large observable polarisation. This
has implications for accreting systems ranging from pulsars and X-ray binaries
to AGN.
