Seals are skillful swimmers and divers. They can swim up to 15 to 18 miles per hour (24 to 29 km/h) and are very agile in the water. The eared seals get their swimming thrust mainly from the forelimbs. The earless seals get most of their thrust from their hind limbs. Seals can dive to great depths—160 to 600 feet (50 to 180 m) is common, but some northern elephant seals have been recorded diving to depths of more than 2,000 feet (610 m). Many species are capable of remaining submerged for 20 minutes, some for more than 30 minutes.

On land, seals are rather clumsy. Eared seals, which can bring their hind limbs under their body, can stand on all fours and manage to move with a kind of gallop. Earless seals cannot move their hind limbs forward; they move on land by using their forelimbs and wriggling their body. Some seals, such as the common, or harbor, seal, spend much of their time on land; others, such as the northern fur seal, may spend up to eight months at sea. Many species spend their time out of the water exclusively on ice (rather than land).

Seals eat fish, crustaceans, squid, and occasionally, birds and other seals. They catch their food primarily with their mouths.

Most species of seals return year after year to the same area—often the place of their birth—for breeding. These breeding grounds are called rookeries. Some species travel great distances each year from their feeding grounds to their rookery. Most seals have well-defined and short breeding seasons so that all the seals of a colony come together at the same time.

All eared and some earless seals are polygamous. The mature bulls attempt to gather a harem of females, and those who succeed are called harem bulls. The size of the harem, ranging from 3 to 40 females, depends on the species and on the strength and ferocity of the bull. Except for the gray and elephant seals, the earless seals are believed to be monogamous.

In polygamous species, the mature bulls are the first to arrive on the breeding grounds, where they attempt to establish definite territories. The females, pregnant from last year's mating, arrive two weeks later. A single pup is born within one week. After the birth, mating takes place. The gestation period is 250 to 365 days. Immature bulls arrive at the rookeries a few weeks after the mature males and remain on the fringes of the colony. A young bull is not sexually mature until four years of age and does not attempt to establish a harem until five to seven years of age.

Most pups (baby seals) are precocial (born in an advanced state of development) and are able to swim soon after birth. A newborn pup is thin, though it appears to be fat because it has a thick coat of fur. The pup grows quickly and, because the mother's milk is 45 per cent fat, it soon develops a thick layer of blubber.