  We review some major, open issues in the current modelling of low and
intermediate mass, main sequence stars based on seismological studies. In the
present paper, the solar case is discussed together with current problems that
are common to the Sun and stars with a structure similar to that of the Sun.
Several additional issues specific to main sequence stars other than the Sun
are reviewed and illustrated with a few stars observed with CoRoT in a
companion paper.
