As with all animals, crustaceans serve as hosts to a very diverse taxa of parasites. These parasites range from unusual dinoflagellates that parasitise the hemocoels or eggs of their hosts, to classical helminths that use crustaceans as intermediate hosts, and to the bizarrely adapted tantulocarid and rhizocephalan crustaceans with their highly derived life styles. Here, I review the major parasitic taxa that use Crustacea as hosts. The parasites of decapods, particularly those in shrimps, crabs, and lobsters are the best known, primarily because of their impact on populations of their commercially important hosts. Several of these parasites are outright pathogens that cause widespread mortality, feminisation, and stunting in their host populations. Other parasites, particularly those in copepods, cladocerans, and amphipods have also received attention because of the ecological importance of these hosts in food webs. They have received notable studies on vertical transmission, the influence of cryptic species complexes (both host and parasite), as well as the emergence of new pathogens in these hosts. A few parasites are also known from brine shrimp (Anostraca) and barnacles (Cirripedia) which have served as laboratory or ecological models, respectively, but few of these parasites have received much study other than their initial taxonomic descriptions and systematic placement. Although molecular tools have revealed the systematics of many of the parasitic taxa, their biology and ecology remain poorly known.