  Most Tidal Dwarf Galaxies (TDGs) so-far discussed in the literature may be
considered as young ones or even newborns, as they are still physically linked
to their parent galaxies by an umbilical cord: the tidal tail at the tip of
which they are usually observed. Old Tidal Dwarf Galaxies, completely detached
from their progenitors, are still to be found. Using N--body numerical
simulations, we have shown that tidal objects as massive as 10^9 solar masses
may be formed in interacting systems and survive for more than one Gyr. Old
TDGs should hence exist in the Universe. They may be identified looking at a
peculiarity of their "genetic identity card": a relatively high abundance in
heavy elements, inherited from their parent galaxies. Finally, using this
technique, we revisit the dwarf galaxies in the local Universe trying to find
arguments pro and con a tidal origin.
