Ship registration is the process by which a ship is documented and given the nationality of the country to which the ship has been documented. The nationality allows a ship to travel internationally as it is proof of ownership of the vessel.
International law requires that every merchant ship be registered in a country, called its flag state. Ship registration is similar to a person receiving a passport. A ship is bound to the law of its flag state. It is usual to say that the ship sails under the flag of the country of registration.
A ship's flag state exercises regulatory control over the vessel and is required to inspect it regularly, certify the ship's equipment and crew, and issue safety and pollution prevention documents. The organization which actually registers the ship is known as its registry. Registries may be governmental or private agencies. In some cases, such as the United States' Alternative Compliance Program, the registry can assign a third party to administer inspections.