Vietnam, Malaysia, Philippines, tiny Brunei, Taiwan and, most significantly, China lay claim to some part of the sea
It has finally been decided that the Philippines has exclusive sovereign right to West Philippine Sea (in the South China Sea) and that China’s “nine-dash line” is invalid, according to the United Nations (UN) Arbitral Tribunal.
China claims most of the sea, even waters approaching neighbouring countries, based on a vaguely defined "nine-dash-line" found on a 1940's Chinese map. The Philippines and other countries dispute this claim.
Commentators say the 3 million square kilometres (1.2 million square miles) of water are a potential flashpoint for regional conflict.
What's there and who's disputing it?
It's mostly empty, and hundreds of the small islands, islets and rocks are not naturally able to support human settlement. Significant chains include the Paracels in the north and the Spratlys in the south.
But everyone surrounding the sea, Vietnam, Malaysia, the Philippines, tiny Brunei, Taiwan and, most significantly, China, lay claim to some part of it.
Why is there any dispute if there's nothing there?
Scientists believe that the seabed could contain unexploited oil, gas and minerals, which would be a boon to any country that can establish their claims to the region's waters, especially in resource-hungry Asia. It's also home to abundant fisheries that feed growing populations.
But the sea's key value is strategic. Shipping lanes vital to world trade pass through it, carrying everything from raw materials to finished products, as well as enormous quantities of oil.
Beijing views the South China Sea as its own backyard, a place where it is entitled to free, uninterrupted rein and where its growing navy should be able to operate unhampered.
How are these disputes playing out?
For years, claimants have been building up the tiny reefs and islets to bolster their claims to ownership. China's land-reclamation programme has been particularly aggressive.
Satellite pictures now show inhabited islands where there was once only submerged coral and many have multiple facilities, including some with runways long enough for huge planes.
Beijing insists its intent is peaceful but the US and others suspect China is trying to assert its sovereignty claims and say that it could pose threats to the free passage of ships.
Washington says the waters are international and regularly sends its warships there on so-called "Freedom of Navigation" missions.
China says these missions are provocations and warns the US not to interfere. It regularly stages its own exercises in the area as a show of force.