The element settling which occurs inside stars, also called "microscopic diffusion", due to the combined effect of gravity, thermal gradient, radiative acceleration and concentration gradients, leads to abundance variations which cannot be neglected in the computations of stellar structure. These processes were first introduced to account for abundance anomalies in "peculiar stars", but there 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. I will give a review of these questions and the constraints obtained on the mixing processes for main sequence stars. I will specially discuss new computations of the coupling between element settling and rotation-induced mixing. I will show that helium gradients induced by microscopic diffusion lead to non-negligible terms in the meridional circulation, which have interesting consequences for the computations of abundance variations in stars. I will also discuss the consequences of these processes on the stellar oscillations.

 
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