  Element settling inside the Sun now becomes detectable from the comparison of
the observed oscillation modes with the results of the theoretical models. This
settling is due, not only to gravitation, but also to thermal diffusion and
radiative acceleration (although this last effect is small compared to the two
others). It leads to abundance variations of helium and heavy elements of
$\cong 10%$ below the convective zone. Although not observable from
spectroscopy, such variations lead to non-negligible modifications of the solar
internal structure and evolution. Helioseismology is a powerful tool to detect
such effects, and its positive results represent a great success for the theory
of stellar evolution. Meanwhile, evidences are obtained that the element
settling is slightly smoothed down, probably due to mild macroscopic motions
below the convective zone. Additional observations of the abundances of both
Solar $^7$Li and $^3$He lead to specific constraints on these particular
motions.
