  In this paper we review the observational results on Relic radio sources in
clusters of galaxies. We discuss their observational properties, structures and
radio spectra. We will show that Relics can be divided according to their size,
morphology, and location in the galaxy cluster. These differences could be
related to physical properties of Relic sources. The comparison with cluster
conditions suggests that Relics could be related to shock waves originated by
cluster mergers.
