any of numerous crustaceans (order Decapoda, phylum Arthropoda) constituting the families Astacidae (Northern Hemisphere), Parastacidae, and Austroastracidae (Southern Hemisphere). They are closely related to the lobster. Over half of the more than 500 species occur in North America. Nearly all live in fresh water, although a few species occur in brackish water or salt water.

Crayfish are characterized by a joined head and thorax, or midsection, and a segmented body, which is sandy yellow, green, red, or dark brown in colour. The head has a sharp snout, and the compound eyes are on movable stalks. The exoskeleton, or body covering, is thin but tough. The front pair of the five pairs of legs have large, powerful pincers (chelae). There are five pairs of smaller appendages on the abdomen, used mostly for swimming and circulating water for respiration.

Most adult crayfish are about 7.5 cm (3 inches) long. Among the smallest is the 2.5-cm-long Cambarellus diminutus of the southeastern United States. Among the largest is Astacopsis gouldi of Tasmania, which may reach 40 cm in length and weigh about 3.5 kg (8 pounds).

Crayfish, common in streams and lakes, often conceal themselves under rocks or logs. They are most active at night, when they feed largely on snails, insect larvae, worms, and amphibian tadpoles; some eat vegetation. Crayfish mate in the autumn and lay eggs in the spring. The eggs, attached to the female's abdomen, hatch in five to eight weeks. The larvae remain on the mother for several weeks. Sexual maturity is achieved in a few months to several years, and the life span ranges from 1 to 20 years, depending on the species.

The most common genera of North America include Procambarus, Orconectes, Faxonella, Cambarus, Cambarellus, and Pacifastacus. Austropotamobius, the most common genus of Europe, is the only one native to Great Britain. The genus Astacus occurs in Europe, the genus Cambaroides in East Asia.