This review presents basic equations for the solution of the NLTE radiative
transfer problem for trace elements and methods for its solution are
summarized. The importance of frequency coupling in radiative transfer in
stellar atmospheres is emphasized.Comment: presented at the workshop held in Nice, France, 30.7.-4.8.2007, to
appear in Non-LTE Line Formation for Trace Elements in Stellar Atmospheres,
R. Monier et al. eds., EAS Publ. Se
