Modelling broad emission lines in active galactic nuclei
  Broad Emission Lines are the most characteristic features of Active Galaxies,
but the mechanism of creating a medium able to emit these intense lines is not
quite clear. Observations clearly indicate that the motion of the material is
predominantly Keplerian, with traces of inflow and clear signatures of outflow,
but this still does not point out whether the lines partially come from the
disk surface, or exclusively from the circumnuclear material, and whether this
material originates from the disk as a wind, or comes, at least partially, from
outside. I review the basic scenarios for the formation of the Broad Line
Region (BLR), and the recent progress in modelling the physical conditions in
the emitting medium. The current state is the the outer radius of the BLR is
fixed by the dust sublimation temperature in the medium exposed to the
irradiation from the central source, the inner radius is likely fixed by the
dust sublimation temperature in the atmosphere of the non-illuminated accretion
disk, and the local cloud density is a universal number fixed by the radiation
pressure confinement. The time-dependent aspects of the BLR formation, however,
still wait for serious modelling effort matching the quality of the
observational data.
