in maritime law, all parts of the mass of saltwater surrounding the globe that are not part of the territorial sea or internal waters of a state. For several centuries beginning in the European Middle Ages, a number of maritime states asserted sovereignty over large portions of the high seas. Well-known examples were the claims of Genoa in the Mediterranean and of Great Britain in the North Sea and elsewhere.

The doctrine that the high seas in time of peace are open to all nations and may not be subjected to national sovereignty (freedom of the seas) was proposed by the Dutch jurist Hugo Grotius (Grotius, Hugo) as early as 1609. It did not become an accepted principle of international law, however, until the 19th century. Freedom of the seas was ideologically connected with other 19th-century freedoms, particularly laissez-faire economic theory, and was vigorously pressed by the great maritime and commercial powers, especially Great Britain. Freedom of the high seas is now recognized to include freedom of navigation, fishing, the laying of submarine cables and pipelines, and overflight of aircraft.

By the second half of the 20th century, demands by some coastal states for increased security and customs zones, for exclusive offshore-fishing rights, for conservation of maritime resources, and for exploitation of resources, especially oil, found in continental shelves caused serious conflicts. The first United Nations Conference on the Law of the Sea, meeting at Geneva in 1958, sought to codify the law of the high seas but was unable to resolve many issues, notably the maximum permissible breadth of the territorial sea subject to national sovereignty. A second conference (Geneva, 1960) also failed to resolve this point; and a third conference began in Caracas in 1973, later convening in Geneva and New York City.