  The uncertainties which still plague our understanding of the evolution of
the light nuclides D, 3He and 4He in the Galaxy are described. Measurements of
the local abundance of deuterium range over a factor of 3. The observed
dispersion can be reconciled with the predictions on deuterium evolution from
standard Galactic chemical evolution models, if the true local abundance of
deuterium proves to be high, but not too high, and lower observed values are
due to depletion onto dust grains. The nearly constancy of the 3He abundance
with both time and position within the Galaxy implies a negligible production
of this element in stars, at variance with predictions from standard stellar
models which, however, do agree with the (few) measurements of 3He in planetary
nebulae. Thermohaline mixing, inhibited by magnetic fields in a small fraction
of low-mass stars, could in principle explain the complexity of the overall
scenario. However, complete grids of stellar yields taking this mechanism into
account are not available for use in chemical evolution models yet. Much effort
has been devoted to unravel the origin of the extreme helium-rich stars which
seem to inhabit the most massive Galactic globular clusters. Yet, the issue of
4He evolution is far from being fully settled even in the disc of the Milky
Way.
