  The UV-X continuum, the X-ray spectral features, and the variability in these
bands provide powerful tools for studying the innermost regions of AGNs from
which we gain an insight into the accretion process. In this chapter the
discussion focusses on luminous AGN, i.e. Seyfert galaxies and quasars. The
standard accretion disk model (a stationary geometrically thin disk) is
described, and vertically averaged solutions for the radial structure are
given. The emission of the standard disk is discussed using different
approximations, and it is compared to the observations. This leads to the
conclusion that more complex models are required, such as the irradiated disk
and the disk-corona models. The advantage of this last model is that it
explains the overall UV-X spectral distribution. In the framework of these disk
models, the profile, intensity, and variability properties of the X-ray iron
line can be explained by reprocessing at the surface of the cold disk very
close to the black hole (the "relativistic disk model"). An alternative
possibility is discussed, where the UV-X continuum is produced by a
quasi-spherical distribution of dense clouds surrounded by (or embedded in) a
hot medium. In such a model the iron line profile could be due to
Comptonization instead of relativistic effects.
