Diffusion and mixing in main-sequence stars

The element settling which occurs inside stars, also called 'microscopic diffusion', due to the combined effect of gravity, thermal gradient, radiative acceleration and concentration gradient, leads to abundance variations which cannot be neglected in the computations of stellar structure. These processes where first introduced to account for abundance anomalies in 'peculiar stars', but their importance in the so-called 'normal' stars is now fully acknowledged, specially after the evidence of helium settling in the Sun from helioseismology. The reason why abundance variations as large as predicted by microscopic diffusion are not always observed is due to the influence of macroscopic motions, like rotation-induced mixing, or mass loss, which increase the settling time scales. In the present review, I discuss the theories of element settling and rotation-induced mixing and the importance of their coupling. I also give some comments about the links between diffusion processes and asteroseismology.