Evolution of D and 3He in the Galaxy

The predictions of Galactic chemical evolution models for D and ${}^3$He are described in connection with those on the other Galactic quantities for which observational constraints are available. Models in agreement with the largest set of data predict deuterium depletions from the Big Bang to the present epoch smaller than a factor of 3 and do not allow for D/H primordial abundances larger than $\sim 4\times {10}^{-5}$. Models predicting higher D consumption do not reproduce other observed features of our Galaxy. If both the primordial D and ${}^3$He are low, models assuming that 90% of low-mass stars experience an extra-mixing during the red giant phase reproduce all the ${}^3$He observed abundances. The same percentage allows to fit also the observed carbon isotopic ratios, thus supporting the self-consistency of the extra-mixing mechanism.