There is a whole class of astronomical objects for which radiation is not only a probe of the physical state, but is to fact an important energy balance agent. In these objects, radiative transfer plays a crucial role. Typical examples of such a situation are stellar atmospheres, interstellar matter and various structures within, and accretion disks, both around stellar mass degenerate objects (cataclysmic variables), and around supermassive black holes (quasars).
We concentrate on two objects - stellar atmospheres and quasar accretion disks, and approach. We will give a brief overview of recent progress in their modeling. On the stellar atmosphere side, very sophisticated non-LTE models including millions of lines are now being constructed. It is shown that they provide and excellent match to high-quality ground- and space-based spectra. In quasar modeling, we have constructed a grid of non-LTE disk models for a wide range of parameters, and begun a study of analysing observed data using the new models.
The properties of the standard accretion disk are summarized. Recent models of steady and time dependent disks are discussed. The two dimensional calculations of accretion flows are examined.
This short review highlights some open questions regarding the dynamics of accretion disks in Algol and related systems. Standard disk theory hes been quite sucessful in cataclysmics, prostellar thick disks, and active galactic nuclei, but our understanding of relatively normal accretion flows remains less satisfactory. Some problems that remain unanswered include the treatment of the Roche surface as a limiting radius, the structure of Algol disks and viscous accretion, and boundary layer.