Decapod crustaceans are familiar to anybody who has ever visited the seashore as a kid or adult, with crabs and shrimps being abundant in rock pools and on beaches. They are however, much more widespread than seashores, and a variety of species live in the deepest ocean trenches, on deep-sea vents, on coral reefs, and in freshwater environments, such as lakes, rivers and in underground caves. They have also colonised land, with some species of leaving far inland and even up trees in tropical rain forests.

Decapods range in size from barely a cm in length, such as the freshwater shrimp Caridella minuta from Lake Tanganyika and the tiny Oriental freshwater crabs of the family Hymenosomatidae to the Giant Japanese Spider crab, Macrocheira kaempferi with a leg span of nearly 4 metres. Other notable giants are the largest terrestrial arthropod, the Coconut Robber, Birgus latro and the largest freshwater invertebrate, the Giant Tasmanian Crayfish, Astacopsis gouldi.