  The interdependence of microscopic (atomic) and macroscopic (hydrodynamic)
processes inside stars and their consequences for stellar structure and
evolution were recognized by Jean-Paul Zahn several decades ago. He was a
pioneer in that respect, discussing the importance of the macroscopic motions
related to stellar rotation, in competition with the chemical stratification
induced by gravitational settling and radiative accelerations. This has been
much developed in recent years, in connection with the improvement of
observational data, including asteroseismology. Morover, it has been recently
discovered that the microscopic atomic diffusion processes can lead to
macroscopic results which may infuence in a non negligible way the internal
stellar structure, independently of the abundances observed at the surface.
