The recent emergence of the green crab, Carcinus maenas, on the west coast has prompted further review of the status and distribution of nonindigenous crustaceans in the United States. Other species of concern include Eriocheir sinensis (Chinese mitten crab), Hemigrapsus sanguineus (Japanese shore crab), Bythotrephes cederstroemi (spiny waterflea), Daphnia lumholtzi (waterflea), Orconectes rusticus (rusty crayfish), Mysis relicta (opossum shrimp), and Peneaus monodon (Asian tiger shrimp). There are least 77 species of crustaceans that are considered nonindigenous to the waters in which they occur. Out of these 77 species, 44 have become established in their new environment. Crustaceans are found in every kind of aquatic habitat. Over 50 of these introduced species are found in marine environments, the rest are found in fresh water. The origins of these nonindigenous crustaceans are global; there is at least one species from every continent and ocean. Many are from Asia, but for some the origins are unknown. Introduction of at least one species has occurred in the inland or coastal waters of 43 states. California ranks the highest with over 50 species of nonindigenous crustaceans, mostly in coastal waters and bays. The higher profiled taxa of nonindigenous decopods includes crabs, crayfish, lobsters, and shrimps, but nearly half of the introduced species are the smaller amphipods and copepods. Introductions of crustaceans as early as 1873, have typically occurred through aquaculture and research escapes and releases, ballast water discharge, ship fouling, stocking for food or gamefish forage, and stock contamination with fish or oyster species. Studies have shown that nonindigenous crustaceans impact food webs and fish communities, exclude native congeners, and alter habitats. The green crab is of special concern because of its potential as a nuisance species. Potential impacts include predation on bivalve populations and competition with native crabs.