  Model atoms are an integral part in the solution of non-LTE problems. They
comprise the atomic input data that are used to specify the statistical
equilibrium equations and the opacities and emissivities of radiative transfer.
A realistic implementation of the structure and the processes governing the
quantum-mechanical system of an atom is decisive for the successful modelling
of observed spectra. We provide guidelines and suggestions for the construction
of robust and comprehensive model atoms as required in non-LTE line-formation
computations for stellar atmospheres. Emphasis is given on the use of standard
stars for testing model atoms under a wide range of plasma conditions.
