Crayfish are any of the many crustaceans that belong to the families Astacidae (Northern Hemisphere), Parastacidae (Southern Hemisphere), and Austroastracidae (Northern Hemisphere) in the order Decapoda, phylum Arthropoda. They have a tight relationship with lobsters. North America is home to more than half of the world's 500 species. The majority of species are found in freshwater environments, with a minority existing in brackish or saltwater. The head and thorax, or midsection, of crayfish are united, and its segmented body is colored in shades of sandy yellow, green, red, or dark brown. The complex eyes are on movable stalks, and the head has a pointed snout. The bodily covering, or exoskeleton, is robust while being thin. Large, strong pincers (chelae) are found on the front pair of the five pairs of legs. The abdomen has five pairs of tiny appendages that are mostly used for swimming and breathing through water circulation. A mature crayfish's length is typically 7.5 cm (3 inches). The 2.5-cm-long Cambarellus diminutus found in the Southeast of the United States is one of the smallest. Tasmania's Astacopsis gouldi, which may grow to 40 cm in length and weigh up to 3.5 kg (8 pounds), is one of the biggest.

A wide range of freshwater environments, including rivers, streams, lakes, dams, and wetlands, are home to crayfish. Being mostly vegetarians, they are typically most active at night (nocturnal), when they forage on the streambed and consume the rotting leaves and water weeds. However, because they are opportunistic omnivores, crayfish can consume a wide variety of foods, including meat. Crayfish can burrow; all of them can. Certain species are better at digging than others. The Land Crayfish, or genus Engaeus, have altered their bodies to live in enclosed spaces and have grown so specialized at burrowing that they spend their whole life cycle in burrows. Among these specializations include a smaller body, typically measuring no more than 45 mm in length, as well as smaller eyes, antennae, and tail. In order to reach the water table, the burrows can reach a length of up to 10 meters, and they are frequently identified by mud chimneys at the entrance. The height of these chimneys might vary from a few centimeters to forty centimeters.