The conditions of thermodynamic equilibrium, local thermodynamic equilibrium,
and statistical equilibrium are discussed in detail. The equations of
statistical equilibrium and the supplementary equations are shown together with
the expressions for radiative and collisional rates with the emphasize on the
solution for trace elements.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
