  The nuclei of the lithium isotopes are fragile, easily destroyed, so that, at
variance with most of the other elements, they cannot be formed in stars
through steady hydrostatic nucleosynthesis. The 7Li isotope is synthesized
during primordial nucleosynthesis in the first minutes after the Big Bang and
later by cosmic rays, by novae and in pulsations of AGB stars (possibly also by
the "nu" process). 6Li is mainly formed by cosmic rays. The oldest (most
metal-deficient) warm galactic stars should retain the signature of these
processes if, (as it had been often expected) lithium is not depleted in these
stars. The existence of a "plateau" of the abundance of 7Li (and of its slope)
in the warm metal-poor stars is discussed. At very low metallicity ([Fe/H]<-2.7
dex the star to star scatter increases significantly towards low Li abundances.
The highest value of the lithium abundance in the early stellar matter of the
Galaxy (A(7Li) = 2.2 dex) is much lower than the the value A(7Li) = 2.72
predicted by the standard Big Bang nucleosynthesis, according to the
specifications found by the satellite WMAP. After gathering a homogeneous
stellar sample, and analysing its behaviour, possible explanations of the
disagreement between Big Bang and stellar abundances are discussed (including
early astration and diffusion). On the other hand, possibilities of lower
productions of 7Li in the standard and/or non-standard Big Bang nucleosyntheses
are briefly evoked. A surprisingly high value (A(6Li)=0.8 dex) of the abundance
of the 6Li isotope has been found in a few warm metal-poor stars. Such a high
abundance of 6Li independent of the mean metallicity in the early Galaxy cannot
be easily explained. But are we really observing 6Li ?
