2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/444A 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.8 pages, 2 figures, LaTeX, uses newpasp.sty, invited talk at IAU 199 `The Universe at Low Radio Frequencies' in Pune, India, 1999Astrophysicstext