An isopod is any member of the order Isopoda. Isopods are segmented crustaceans, the best known of which are woodlice, pill bugs, sow bugs, and other land-dwelling species. In caves, aquatic isopods are often studied because many are troglobitic and depend on caves for survival. Most isopods are small; however, there are about nine species of so-called giant isopods in the genus Bathynomus.

The sea slater or rock slater, a crustacean of the order Isopoda, which also includes woodlice, has a simple, typically arthropod structure. Its armoured exoskeleton is jointed between the body segments, and its seven pairs of legs, borne on the thorax, are all very similar in proportions (Isopoda means "equal feet"). Appendages beneath the abdomen function as gills