  Active Galactic Nuclei are powered by accretion of matter onto a supermassive
black hole (SMBH) of mass Mbh ~ 10^{5}-10^{9} Msun. The accretion process is
indeed the most efficient mechanism for energy release we currently know of,
with up to ~30-40% of the gravitational rest mass energy that can be converted
into radiation. The vast majority of this energy is released at high energy
(UV-X-rays) within the central $100$ gravitational radii from the central SMBH.
This energy release occurs through a variety of emission and absorption
mechanisms, spanning the entire electromagnetic spectrum. The UV emission being
commonly explained by the presence of an optically thick accretion flow, while
the X-rays generally require a hotter, optically thinner, plasma, the so-called
X-ray corona. If outflows are present, they can also extract a significant part
of the gravitational power. With an origin in the deep potential well of the
SMBH, the study of the high-energy emission of AGN give a direct insight into
the physical properties of the accretion, ejection and radiative mechanisms
occurring in the SMBH close environment. While not exhaustive, we discuss in
this chapter our present understanding of these mechanisms, the limitations we
are currently facing and the expected advances in the future.
