  Helioseismology, the study of global solar oscillations, has proved to be an
extremely powerful tool for the investigation of the internal structure and
dynamics of the Sun. Studies of time changes in frequency observations of solar
oscillations from helioseismology experiments on Earth and in space have shown,
for example, that the Sun's shape varies over solar cycle timescales. In
particular, far-reaching inferences about the Sun have been obtained by
applying inversion techniques to observations of frequencies of oscillations.
The results, so far, have shown that the solar structure is remarkably close to
the predictions of the standard solar model and, recently, that the
near-surface region can be probed with sufficiently high spatial resolution as
to allow investigations of the equation of state and of the solar envelope
helium abundance. The same helioseismic inversion methods can be applied to the
rotational frequency splittings to deduce with high accuracy the internal
rotation velocity of the Sun, as function of radius and latitude. This also
allows us to study some global astrophysical properties of the Sun, such as the
angular momentum, the grativational quadrupole moment and the effect of
distortion induced on the surface (oblateness). The helioseismic approach and
what we have learnt from it during the last decades about the interior of the
Sun are reviewed here.
