Hydrogen lines

Advances in modern detectors have allowed hydrogen Balmer lines to be used as high-precision diagnostics of effective temperature in F, G and K stars. Their precision as such a diagnostic is now to a large degree dependent on our understanding of their formation in stellar atmospheres. In this paper, I review the current status of Balmer lines as effective temperature diagnostics and in particular discuss two aspects of their line formation: (i) the question of whether the line wings form in Local Thermodynamic Equilibrium (LTE) or not and (ii) the broadening of the line wings by collisions.