The Common Shag (Phalacrocorax aristotelis) is a member of the cormorant family of seabirds. It breeds around the coasts of western and southern Europe and north Africa, mainly wintering in its breeding range except for northernmost birds. In Britain it is usually referred to as the Shag; in scientific literature it is sometimes called the European Shag.

It breeds mainly on coasts, nesting in crevices or small caves. Three eggs are laid in a nest of seaweed or twigs.

This is a large black bird, 68-78 cm long and with a 95-110 cm wingspan. It has a longish tail and yellow throat-patch. Adults have a small crest in the breeding season. It is distinguished from the Great Cormorant by its smaller size, lighter build, thinner bill, and, in breeding adults, by the crest and green-tinged plumage.

It feeds on the sea, and, unlike Great Cormorant, is rare inland. It will winter along any coast that is well-supplied with fish.

The Shag is one of the deepest divers among the cormorant family. Using depth gauges, Shags have been shown to dive to at least 45 metres. Shags are preponderantly benthic feediers, i.e. they find their prey on the sea bottom. They will eat a wide range of fish but their commonest prey is the sand eel. Shags will travel many kilometres from their roosting sites in order to feed.

In UK coastal waters, dive times are typically around 45 seconds, with a recovery time of around 15 seconds between dives; this is consistent with aerobic diving , i.e. the bird depends on the oxygen in its lungs and dissolved in its bloodstream during the dive.