  Black Holes generate a particular kind of environments dominated by an
accretion flow which concentrates a magnetic field. The interplay of gravity
and magnetism creates this paradoxical situation where relativistic ejection is
allowed and consequently high energy phenomena take place. Therefore Black
Holes, which are very likely at the origin of powerfull astrophysical phenomena
such as AGNs, micro- quasars and GRBs where relativistic ejections are
observed, are at the heart of high energy astrophysics. The combination of
General Relativity and Magneto-HydroDynamics (MHD) makes theory difficult;
however great pionneers opened beautiful tracks in the seventies and left
important problems to be solved for the next decades. These lectures will
present the status of these issues. They have a tutorial aspect together with
critical review aspect and contain also some new issues. Most of these lectures
has been presented at the "School on Black Hole in the Universe" at Cargese, in
May 2003.
