  We investigate the stellar populations of a sample of Tidal Dwarf Galaxies,
combining observations and evolutionary synthesis models to try and reveal
their formation mechanism. On optical images we select a first sample of TDGs
for which optical spectroscopy is used to measure metallicities and velocity
structure. Finally, we estimate ages, burst strengths, and stellar masses from
near-infrared imaging in comparison with a dedicated grid of evolutionary
synthesis models, to assess if Tidal Dwarfs are formed out of collapsing gas
clouds or by an accumulation of old stars from the parent galaxy or by a
combination of both.
