Formation of Non-LTE spectral lines and their impacts on red giant stars

Stellar abundance analyses generally rely on the Local Thermodynamic Equilibrium (LTE) assumption for spectral line formation. This assumption is not always appropriate, in particular for metal-poor and/or evolved stars. In this presentation, I will give the basics to understand the non-LTE spectral line formation and expose the expected impacts illustrated by examples. Then, scientific cases based on this approach will be given from individual red giants to stellar populations. The consequences in the era of the Gaia mission and the large spectroscopic surveys will be underlined with a special focus on stellar atmospheric parameters.