  Lithium is an excellent tracer of mixing in stars as it is destroyed (by
nuclear reactions) at a temperature around $\sim 2.5\times 10^6$ K. The lithium
destruction zone is typically located in the radiative region of a star. If the
radiative regions are stable, the observed surface value of lithium should
remain constant with time. However, comparison of the meteoritic and
photospheric Li abundances in the Sun indicate that the surface abundance of Li
in the Sun has been depleted by more than two orders of magnitude. This is not
predicted by solar models and is a long standing problem. Observations of Li in
open clusters indicate that Li depletion is occurring on the main sequence.
Furthermore, there is now compelling observational evidence that a spread of
lithium abundances is present in nearly identical stars. This suggests that
some transport process is occurring in stellar radiative regions. Helioseismic
inversions support this conclusion, for they suggest that standard solar models
need to be modified below the base of the convection zone. There are a number
of possible theoretical explanations for this transport process. The relation
between Li abundances, rotation rates and the presence of a tidally locked
companion along with the observed internal rotation in the Sun indicate that
the mixing is most likely induced by rotation. The current status of
non-standard (particularly rotational) stellar models which attempt to account
for the lithium observations are reviewed.
