Theoretical Implications of Diffuse Non-Thermal Emission from Clusters   of Galaxies
  A short review on theoretical implications of non-thermal emission (radio, extreme ultraviolet, high energy X-ray) from the intra-cluster medium is given. The origin of cluster radio halos and cluster radio relics is discussed within the framework of a network of processes producing a non-thermal electron population. Emphasis is given to the role of old, remnant, presently invisible relativistic plasma released by former radio galaxies. 
Torsten A. Ensslin (MPI fuer Astrophysik, Garching)