An order of about 4,000 species of dorsoventrally compressed crustaceans that have abdominal gills and similar abdominal and thoracic segments. Terrestrial pillbugs and thousands of benthic marine species are included. Most species are scavengers and/or omnivores; a few are parasitic.

Isopods are one of the most diverse orders of crustaceans, with many species living in all environments, and are common in shallow marine waters. Unlike most crustaceans, isopods are successful on land (suborder Oniscidea, woodlice, which also includes sea slaters), although their greatest diversity remains in the deep sea (suborder Asellota). There are also several parasitic species within the genus Cymothoa, commonly known as "tongue-biters" for their habit of nibbling off fishes' tongues and attaching themselves to the floor of the host's mouth.

An order of sessile-eyed Crustacea, usually having seven pairs of legs, which are all similar in structure.