  The element settling due to the combined effects of gravity, thermal
gradient, radiative acceleration and concentration gradient may lead to
important abundance variations inside the stars, 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. These microscopic
processes work in competition with macroscopic motions, like rotation-induced
mixing or mass loss, which increase the settling time scales. We have recently
obtained clear evidences that asteroseismology of main sequence solar type
stars can give signatures of the chemical variations inside the stars and help
for a better understanding of these processes.
