Role of input atomic data in spectroscopic analyses of the Sun and metal-poor stars

Analysis of high-resolution stellar spectra relies heavil y upon atomic data. These include energy levels, wavelengths, cross-sections for va rious types of interactions between particles and photons, such as photoionization and collision induced transitions. Quantum-mechanical calculations provide largest part of these data. In this paper I describe atomic data necessary to compute model atmospheres and line formation for the late-type stars both in the assumption of local thermodynamic equilibrium (LTE) and in a more general case of non-LTE. I will focus on transition metals with 21< Z< 29 and discuss whether (and where) more complete and/or accurate atomic data are necessary.