  Plasma containing relativistic particles appears in various forms in the
inter galactic medium (IGM): As radio plasma released by active radio galaxies,
as fossil radio plasma from former radio galaxies - so called `radio ghosts',
as `cluster radio relics' in some clusters of galaxies, and as `cluster radio
halos'. The impact of the different forms of radio plasma on the IGM and their
use as diagnostic tools are briefly discussed.
