  The observations of light elements in the Sun and Solar type stars give
special clues for understanding the hydrodynamical processes at work in stellar
interiors. In the Sun 7Li is depleted by 140 while 3He has not increased by
more than 10% in 3 Gyrs. Meanwhile the inversion of helioseismic modes lead to
a precision on the sound velocity of about .1%. The mixing processes below the
solar convection zone are constrained by these observations. Lithium is
depleted in most Pop I solar type stars. In halo stars however, the lithium
abundance seems constant in the "spite plateau" with no observed dispersion,
which is difficult to reconcile with the theory of diffusion processes. In the
present paper, the various relevant observations will be discussed. It will be
shown that the mu-gradients induced by element settling may help solving the
"lithium paradox".
