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Dolphins are a widely distributed and diverse group of aquatic mammals. They are an informal grouping within the order Cetacea excluding whales and porpoises, so to zoologists the grouping is paraphyletic. The comprise the extant families Delphinidae Platanistidae Iniidae and Pontoporiidae and the extinct Lipotidae. There are 40 extant species of dolphins. Dolphins alongside other cetaceans belong to the clade Cetartiodactyla with even-toed ungulates. Cetaceans closest living relatives are the hippopotamuses having diverged about 40 million years ago.

Dolphins range in size from the 1.7 m long and 50 kg Maui's dolphin to the 9.5 m and killer whale. Several species exhibit sexual dimorphism in that the males are larger than females. They have streamlined bodies and two limbs that are modified into flippers. Though not quite as flexible as seals, some can travel at 55.5 km/h. Dolphins use their conical shaped teeth to capture fast moving prey. They have well-developed hearing which is adapted for both air and water and is so well developed that some can survive even if they are blind. Some species are well adapted for diving to great depths. They have a layer of fat or blubber under the skin to keep warm in the cold water.

Although dolphins are widespread, most species prefer the warmer waters of the tropic zones but some like the right whale dolphin prefer colder climates. Dolphins feed largely on fish and squid but a few like the killer whale feed on large mammals like seals. Male typically mate with multiple females every year but females only mate every two to three years. Calves are typically born in the spring and summer months and females bear all the responsibility for raising them. Mothers of some species fast and nurse their young for a relatively long period of time. Produce a variety of vocalizations usually in the form of clicks and whistles.

Dolphins are sometimes hunted in places like Japan in an activity known as dolphin drive hunting. Besides drive hunting, they also face threats from bycatch habitat loss and marine pollution. Dolphins have been depicted in various cultures worldwide. Dolphins occasionally feature in literature and film as in the film series Free Willy. Dolphins are sometimes kept in captivity and trained to perform tricks. The most common dolphins kept are killer whales and bottlenose.

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