Active Galactic Nuclei are powered by accretion of matter onto a supermassive black hole (SMBH) of mass M BH ∼ 10 5−9 M . 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.