An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations. They have evolved from wooden vessels, used to deploy balloons, into nuclear-powered warships that carry dozens of fixed- and rotary-wing aircraft.
Aircraft carriers are typically treated as the capital ship of a fleet and are extremely expensive to build and important to protect: of the nine nations which possess an aircraft carrier, six of these navies only possess one such ship. There are 20 active aircraft carriers in the world as of June 2011.