There are at least 32 species of dolphins. Typical examples are the bottle-nosed dolphin, a popular performer in seaquariums, and the common dolphin, which inspired many Mediterranean folk legends. Both often appear in open waters, making their characteristic arched bounds, frequently before the bow waves of ships. Several freshwater species inhabit river estuaries in Asia and South America. The small, graceful tucuxi dolphin has been sighted more than 2000 km (more than 1250 mi) up the Amazon River. The tucuxi, the smallest dolphin, is less than 1.2 m (less than 4 ft) long; the largest, the bottle-nosed dolphin, reaches a length of 3 m (10 ft). The killer whale is considered a dolphin despite its much greater length of 9 m (30 ft). The pilot whale is also considered a type of dolphin.

Dolphins once were hunted commercially, especially for the small quantity of valuable oil extracted from parts of the head and used to lubricate delicate watch mechanisms. Cheaper oils have now been found from other sources, and dolphins are no longer hunted for this reason. Many dolphins, however, become accidentally trapped and drowned in tuna nets; between 1959 and 1972 an estimated 4.8 million dolphins died in this way. Under pressure from animal rights activists and United States consumers, both domestic and international tuna canners have refused to accept shipments from fishing fleets that do not protect dolphins. Concern has also been expressed about the treatment of dolphins on display in public aquariums and in "swim with the dolphins" programs. The Marine Mammal Protection Act of 1972, amended in 1988 and 1992, was passed to prevent exploitation of dolphins and related aquatic animals. The National Marine Fisheries Service is the principal regulatory agency.

Bottlenose dolphin (Tursiops truncatus) average 0.9 to 1.2 m at birth and weigh 8 - 9 kg. Maximum size reported is 3.9 m and 275 kg. Males are sexually mature at 10 - 12 years of age, females between 5 & 12 years. Once reproductively active, females bear a single calf every second or third year. Gestation is about 12 months. Calves are nursed for a year or more. Their maximum age appears to be between 46 - 48 years based on tooth "growth layer group" analysis - both from wild and captive dolphin studies. This species appears to be very "catholic" in their feeding habits, taking a wide variety of fish and invertebrates.