Submarine, a vessel designed to operate below the surface of the water. Submarines were of major importance in both World Wars and today rank with aircraft carriers as the most important ships in the world's navies. The development of nuclear-powered submarines, which have unlimited range and can cruise at high speeds underwater, has revolutionized naval strategy by placing the main emphasis on undersea, rather than surface, warfare. Non-naval submarines are important tools in ocean research and exploration of the ocean floor.

Research submarines are generally called submersibles and are not discussed in this article. (Technically, a submersible is any underwater vessel; a submarine is a submersible that operates without being tended on the surface; and a true submarine is a submarine that is more efficient underwater than on the surface and does not require periodic contacts with the atmosphere to obtain oxygen for the crew.)

Russia has the largest submarine fleet in the world, and about half of it is nuclear powered. The United States has fewer submarines, and all of them are nuclear powered. The U.S. Navy is a leader in the development of noise-suppression technology and of weapons for antisubmarine warfare (ASW). Many other countries include submarines in their naval fleets. These countries include not only major Western powers like Great Britain, France, and Germany, but also less-developed nations like Singapore, Oman, Indonesia, and Saudi Arabia.

Nuclear submarines are of two basic types—attack submarines and ballistic-missile submarines. Conventional submarines are also of two basic types—the long-range cruiser submarines and the small, short-range coastal submarines.

The U.S. Navy names its attack submarines after sea creatures, cities, states, and notable persons. Its ballistic missile submarines are named after states.