Pinnipeds have streamlined, spindle-shaped bodies with reduced or non-existent external ear flaps, rounded heads, flexible necks, limbs modified into flippers, and small tails.
Pinniped skulls have large eye orbits, short snouts and a constricted interorbital region.
They are unique among carnivorans in that their orbital walls are significantly shaped by the maxilla and are not limited by certain facial bones.
Compared to other carnivorans, their teeth tend to be fewer in number (especially incisors and back molars), are pointed and cone-shaped, and lack carnassials.
The walrus has unique upper canines that are elongated into tusks.
The mammary glands and genitals of pinnipeds can retract into the body.

Pinnipeds range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal.
Overall, they tend to be larger than other carnivorans; the southern elephant seal is the largest carnivoran.
Several species have male-biased sexual dimorphism that correlates with the degree of polygyny in a species: highly polygynous species like elephant seals are extremely sexually dimorphic, while less polygynous species have males and females that are closer in size.
In lobodontine seals, females are slightly larger than males.
Males of sexually dimorphic species also tend to have secondary sex characteristics, such as the prominent proboscis of elephant seals, the inflatable red nasal membrane of hooded seals and the thick necks and manes of otariids.
Despite a correlation between size dimorphism and the degree of polygyny, some evidence suggests that size differences between the sexes originated due to ecological differences and prior to the development of polygyny.

Almost all pinnipeds have fur coats, the exception being the walrus, which is only sparsely covered.
Even some fully furred species (particularly sea lions) are less haired than most land mammals.
In species that live on ice, young pups have thicker coats than adults.
The individual hairs on the coat, known collectively as lanugo, can trap heat from sunlight and keep the pup warm.
Pinnipeds are typically countershaded, and are darker colored dorsally and lighter colored ventrally, which serves to eliminate shadows caused by light shining over the ocean water.
The pure white fur of harp seal pups conceals them in their Arctic environment.
Some species, such as ribbon seals, ringed seals and leopard seals, have patterns of contrasting light and dark coloration.
All fully furred species molt; phocids molt once a year, while otariids gradually molt all year.
Seals have a layer of subcutaneous fat known as blubber that is particularly thick in phocids and walruses.
Blubber serves both to keep the animals warm and to provide energy and nourishment when they are fasting.
It can constitute as much as 50% of a pinniped's body weight.
Pups are born with only a thin layer of blubber, but some species compensate for this with thick lanugos.

Pinnipeds have a simple stomach that is similar in structure to terrestrial carnivores.
Most species have neither a cecum nor a clear demarcation between the small and large intestines; the large intestine is comparatively short and only slightly wider than the small intestine.
Small intestine lengths range from 8 times (California sea lion) to 25 times (elephant seal) the body length.
The length of the intestine may be an adaptation to frequent deep diving, as the increased volume of the digestive tract serves as an extended storage compartment for partially digested food during submersion.
Pinnipeds do not have an appendix.
As in most marine mammals, the kidneys are divided into small lobes and can effectively absorb water and filter out excess salt.
Pinnipeds have streamlined, spindle-shaped bodies with reduced or non-existent external ear flaps, rounded heads, flexible necks, limbs modified into flippers, and small tails. Pinniped skulls have large eye orbits, short snouts and a constricted interorbital region. They are unique among carnivorans in that their orbital walls are significantly shaped by the maxilla and are not limited by certain facial bones. Compared to other carnivorans, their teeth tend to be fewer in number (especially incisors and back molars), are pointed and cone-shaped, and lack carnassials. The walrus has unique upper canines that are elongated into tusks. The mammary glands and genitals of pinnipeds can retract into the body.
Pinnipeds range in size from the 1 m (3 ft 3 in) and 45 kg (99 lb) Baikal seal to the 5 m (16 ft) and 3,200 kg (7,100 lb) southern elephant seal. Overall, they tend to be larger than other carnivorans; the southern elephant seal is the largest carnivoran. Several species have male-biased sexual dimorphism that correlates with the degree of polygyny in a species: highly polygynous species like elephant seals are extremely sexually dimorphic, while less polygynous species have males and females that are closer in size. In lobodontine seals, females are slightly larger than males. Males of sexually dimorphic species also tend to have secondary sex characteristics, such as the prominent proboscis of elephant seals, the inflatable red nasal membrane of hooded seals and the thick necks and manes of otariids. Despite a correlation between size dimorphism and the degree of polygyny, some evidence suggests that size differences between the sexes originated due to ecological differences and prior to the development of polygyny.
Almost all pinnipeds have fur coats, the exception being the walrus, which is only sparsely covered. Even some fully furred species (particularly sea lions) are less haired than most land mammals. In species that live on ice, young pups have thicker coats than adults. The individual hairs on the coat, known collectively as lanugo, can trap heat from sunlight and keep the pup warm. Pinnipeds are typically countershaded, and are darker colored dorsally and lighter colored ventrally, which serves to eliminate shadows caused by light shining over the ocean water. The pure white fur of harp seal pups conceals them in their Arctic environment. Some species, such as ribbon seals, ringed seals and leopard seals, have patterns of contrasting light and dark coloration. All fully furred species molt; phocids molt once a year, while otariids gradually molt all year. Seals have a layer of subcutaneous fat known as blubber that is particularly thick in phocids and walruses. Blubber serves both to keep the animals warm and to provide energy and nourishment when they are fasting. It can constitute as much as 50% of a pinniped's body weight. Pups are born with only a thin layer of blubber, but some species compensate for this with thick lanugos.
Pinnipeds have a simple stomach that is similar in structure to terrestrial carnivores. Most species have neither a cecum nor a clear demarcation between the small and large intestines; the large intestine is comparatively short and only slightly wider than the small intestine. Small intestine lengths range from 8 times (California sea lion) to 25 times (elephant seal) the body length. The length of the intestine may be an adaptation to frequent deep diving, as the increased volume of the digestive tract serves as an extended storage compartment for partially digested food during submersion. Pinnipeds do not have an appendix. As in most marine mammals, the kidneys are divided into small lobes and can effectively absorb water and filter out excess salt.
Pinniped