  Galactic collisions trigger a number of phenomena, such as transportation
inward of gas from distances of up to kiloparsecs from the center of a galaxy
to the nuclear region, fuelling a central starburst or nuclear activity. The
inverse process, the ejection of material into the intergalactic medium by
tidal forces, is another important aspect and can be studied especially well
through detailed HI observations of interacting systems which have shown that a
large fraction of the gaseous component of colliding galaxies can be expelled.
Part of this tidal debris might fall back, be dispersed throughout the
intergalactic medium or recondense to form a new generation of galaxies: the
so-called tidal dwarf galaxies. The latter are nearby examples of galaxies in
formation. The properties of these recycled objects are reviewed here and
different ways to identify them are reviewed.
