Lobsters have 20 pairs of gills attached to the bases of the legs and to the sides of the body; the gills are protected by the carapace, the large area of the exoskeleton covering the back and sides of the cephalothorax. In addition to the legs, the appendages consist of 2 paired antennae, 6 pairs of mouth parts, and the small swimmerets attached to the abdominal segments. In the female the eggs remain attached to the swimmerets for 10 or 11 months until they hatch into free-swimming larvae.

The larvae swim for about a year, molting between 14 and 17 times before they settle to the bottom and begin to take on adult characteristics. Lobsters crawl briskly over the ocean floor and swim backward with great speed by scooping motions of the muscular abdomen and tail, but are clumsy on land. They are scavengers but also prey on shellfish and may even attack live fish and large gastropods. Over a period of five years they grow to an average weight of 3 lb (1.4 kg).

There are more than 100 varieties of lobster. The common American lobster, Homarus americanus, is found inshore in summer and in deeper waters in winter from Labrador to North Carolina, but especially along the New England coast, where the chief lobster fisheries are located. Lobsters are caught in slatted wooden traps, or "pots," baited with dead fish. Although protected by law and raised by several hatcheries on the New England coast, they are still in danger of extinction. In Europe a species of Homarus similar to the American is found, but the smaller, less closely related Norway lobster or Dublin prawn, Nephrops norvegicus, is more important commercially.

The spiny, or rock, lobsters, found in warm seas of both hemispheres, are actually marine crayfish (genus Panulirus); they lack claws but have sharp spines on the carapace. The stout-bodied, sometimes brightly colored squat lobsters are close relatives of the hermit crab; their broad abdomens are usually tucked under their bodies, as in crabs, but can be extended and used for backward swimming, as in the true lobsters. True lobsters are classified in the phylum Arthropoda, subphylum Crustacea, order Decapoda, family Nephropidae or Homaridae.