Shearwaters are medium-sized long-winged seabirds. There are more than 20 species of shearwaters, a few larger ones in the genus Calonectris and many smaller species in the genus Puffinus. The Procellaria petrels were formerly believed to belong to this group, but are only distantly related based on more recent studies, while the Pseudobulweria and Lugensa "petrels" are more closely related (Bretagnolle et al., 1998; Nunn & Stanley, 1998). The genus Puffinus can be divided into a group of small species close to Calonectris and a few larger ones more distantly related to both (Austin, 1996).

These birds are most common in temperate and cold waters. They are pelagic outside the breeding season.

These tubenose birds fly with stiff wings and use a “shearing” flight technique to move across wave fronts with the minimum of active flight. Some small species, like Manx Shearwater are cruciform in flight, with their long wings held directly out from their bodies.

Many are long-distance migrants, perhaps most spectacularly Sooty Shearwaters, which cover distances in excess of 14,000 km from their breeding colony on the Falkland Islands (52°S 60°W) north to 65°-70°N in the North Atlantic Ocean off north Norway. Short-tailed Shearwaters perform an even longer "figure of 8" loop migration in the Pacific Ocean from Tasmania to as far north as the Arctic Ocean off northwest Alaska.

They are also extraordinarily long-lived. A Manx Shearwater breeding on Copeland Island, Northern Ireland, is currently (2003/2004) the oldest known wild bird in the world: ringed as an adult (at least 5 years old) in July 1953, it was retrapped in July 2003, at least 55 years old. Manx Shearwaters migrate over 10,000 km to South America in winter, using waters off southern Brazil and Argentina, so this bird has covered a minimum of 1,000,000 km on migration alone.

Shearwaters come to islands and coastal cliffs only to breed. They are nocturnal at the colonial breeding sites, preferring moonless nights to minimize predation. They nest in burrows and often give eerie contact calls on their night-time visits. They lay a single white egg.

They feed on fish, squid, and similar oceanic food. Some will follow fishing boats to take scraps, notably Sooty Shearwater; these species also commonly follow whales to feed on fish disturbed by them.

Shearwaters are part of the family Procellariidae, which also includes fulmars, prions, and petrels.