Chapter 39 of The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon. Fourth Edition, Completely Revised and Expanded [In press, 2007]The Amphipoda have been divided into the suborders Gammaridea, Caprellidea, Cyamidea, Hyperiidea and Ingolfiellidea (Schram 1986, Crustacea. Oxford University Press, New York). However, Myers and Lowry (2003) regard the caprellids, or skeleton shrimps, and the cyamids, or whale lice, as families Caprellidae and Cyamidae. These distinctive groups are covered in separate sections in this manual, for ease of recognition and identification. The Caprellidae (plate 254A) occur on solid surfaces and are strictly marine or estuarine. The Cyamidae are ectoparasites of cetaceans and are occasionally found on beached whales and dolphins (plate 254B). The Hyperiidea (plate 254C) are parasites and commensals of marine macrozooplankton and are exclusively pelagic. Hyperiids are occasionally discovered free swimming intertidally or in shallow-water plankton tows, or are found attached beneath or embedded in the bells of stranded medusae or salps. The Gammaridea (scuds, landhoppers, and beachhoppers) (plate 254E) are the most abundant and familiar amphipods. They occur in pelagic and benthic habitats of fresh, brackish, and marine waters, the supralittoral fringe of the seashore, and in a few damp terrestrial habitats and are difficult to overlook. The wormlike, 2- mm-long interstitial Ingofiellidea (plate 254D) has not been reported from the eastern Pacific, but they may slip through standard sieves and their interstitial habitats are poorly sampled.