  Most of the luminosity of accreting black hole is emitted in the X-ray band.
This radiation is believed to emerge, through inverse Compton process, from a
hot (Te ~ 10^8 -10^9 K) optically thin (Thomson optical depth ~ 1) plasma
probably located in the immediate vicinity of the black hole. The mechanisms at
work in this so called Compton corona can be unveiled through hard X-ray
observations which have revealed a rich phenomenology. Depending on luminosity
different spectral states are observed suggesting that the nature and geometry
of the corona depends on mass accretion rate. In many instances the spectral
behaviour as a function of luminosity shows some degree of hysteresis. The
mechanisms triggering the transition between spectral states is very unclear
although it could be related to an evaporation/condensation equilibrium in an
accretion disc corona system. From the observation of correlation between the
X-ray and radio band, it appears that the Compton corona is intimately related
to the formation of compact jets and probably constitutes the base of the jet.
