Diseases of echinoderms are poorly documented. Most reports concern biotic diseases caused by animal agents. While parasites on echinoderms have been described in increasing numbers of papers for more than one century, the host-parasite relationship and the effects of parasitism on echinoderm life-cycles were rarely considered. The parasitic fauna differs markedly according to the echinoderm group concerned, depending on various factors such as feeding-habits or symbiogenesis. Microorganismic diseases undoubtedly occur in echinoderms but they were not investigated until recently. Microorganisms have frequently been assumed to act as agents causing mass mortalities. As for stress-caused diseases, the only — and very preliminary — data available concern almost exclusively those induced by pollution. Since echinoderms are major components of benthic ecosystems, echinoderm diseases may be expected to exert prominent ecological effects.