Résumé
Due to nuclear reactions within their interior, stars are the generators of the chemical evolution of galaxies. In this context, it is important to recognize that the chemical abundances observed in stellar surfaces are most often not the original ones. In fact, due to atomic diffusion, the atomic species in stars move either downwards or upwards. For solar metallicity stars, it is well accepted within the international community that stellar modeling should include atomic diffusion. However it is not yet considered as a standard process in lower metallicity stars. In this paper I will present the effect of atomic diffusion on surface abundances and on abundance profile in metal poor stars. The effect of the initial metallicity will also be addressed.