  Improvements induced by helioseismology show that microscopic diffusion
cannot be neglected in solar models. Helium and metal diffusion is now
generally introduced in the computations. They cannot however explain the
observed lithium depletion. Macroscopic motions are needed to bring up to the
convection zone the nuclearly depleted matter. Such motions also lead to a
better agreement between the sound velocity in the model and that deduced from
the inversion of seismic modes. On the other hand, they must not bring up too
much $^3$He, as we now know that the $^3$He/$^4$He ratio cannot have changed by
more than 10 percent during these last 3 Gyr. Here we present new consistent
solar models in which the $^3$He constraint is satisfied. The fact that $^3$He
must remain constant while $^7$Li must be depleted leads to strong constraints
on the effective macroscopic diffusion coefficient allowed below the solar
convection zone.
