  Various types of active galactic nuclei (AGN) are briefly discussed, with an
emphasis on the theory of recent X-ray and $\gamma$-ray observations of the
subclass, Seyfert 1 galaxies. The large radiation power from AGN is thought to
originate from gravitational power released by matter accreting onto a
supermassive black hole. The physical mechanisms responsible for the energy
release and the geometry of the gaseous components are still uncertain in spite
of three decades of observational and theoretical studies. The recent X-ray and
gamma-ray observations, however, start to provide useful constraints on the
models. This kind of interpretation of the observations is possible due to
theoretical developments during the last decade of radiative transfer of X-rays
in both optically thick and thin media of various geometries. Particular
attention is paid to various accretion disk-corona models. Recent work on exact
radiative transfer in such geometries are reviewed.
