Cnidarians, especially skeleton-bearing anthozoans and hydrocorals, are known to host abundant and diverse symbiotic fauna encompassing members of the majority of metazoan taxa, ranging from sponges and fl at worms to fi shes. Members of the class Polychaeta are between the most diverse and perhaps the least studied taxa of coral symbionts. The last revision (Martin and Britayev, Oceanogr Mar Biol 36:217–340, 1998) reckoned about 60 species of symbiotic polychaetes associated with more than 100 species of cnidarian hosts. However, this number is considerably underestimated. Some populations of scleractinians, sea fans and black corals show up to 100 % infestation by symbiotic polychaetes. Close association and inter-relation of highly host-specifi c symbionts and cnidarian hosts often lead to dramatic changes in the host morphology. At the moment, actual mechanisms of most of mutual relations between host and symbiont in such associations are generally unknown. The objective of the present paper is to summarize data on species composition and ecology of polychaetes associated with cnidarians. In our review, we report 281 species of cnidarian hosts involved in 324 relationships with symbiotic polychaetes. Most polychaete- hosting cnidarians belong to skeleton-bearing taxa, particularly Scleractinia (125 species or 44.48 % of the total cnidarian hosts), Alcyonaria (73 species or 25.97 %) and Hydrozoa (60 species or 21.35 %). About 120 species of symbiotic polychaetes of ten families are reported from cnidarian hosts. Polynoidae include the highest number of cnidarianassociated polychaetes (almost one half of the currently known species), followed by Syllidae and Serpulidae. Host symbiont interrelations, host specifi city, location, infestation characteristics and adaptive modifi cations of symbionts, as well as host reaction on symbionts presence, have been considered. Our review highlights that (1) every group of cnidarians seems to have their own assemblage of symbiotic polychaetes, (2) some deep-sea alcyonaceans and black corals have never been reported without their often undetermined polynoid symbionts so that its presence has been considered as a species-specifi c, robust taxonomic character, and (3) we certainly expect the polychaete symbionts associated with deep-sea corals to be a hidden hot-spot of diversity, with many species still waiting to be described. The research was supported by Russian Foundation of Basic Researches (grant No 13-04-01332) for T.N. Molodzova and Russian Science Foundation (grant No 14-14-01179) for T.A. Britayev. This review is also a contribution of D. Martin to the Research Project MARISYMBIOMICS (CTM2013-43287-P), funded by the Spanish State Research Plan, and to the Consolidated Research Group on Marine Benthic Ecology of the Generalitat de Catalunya (2014SGR120). Peer reviewed