At the beginning of the 20th century, submarines were small, limited in range and probably lethal to their crews as they were to the enemy. At the beginning of the 21st century, submarines are among the most capable and expensive units of the fleet. The author investigates submarine warfare, from the early days of the 20th century to the beginning of the 21st century, examining aspects of submarine life and operations, covering each topic in depth and allowing comparisons to be made between early and modern methods. Supported by a comprehensive selection of photographs, the author's narrative looks in detail at life below deck; the construction of submarines; means of navigation and communication; weaponry and tactics; submarine aircraft carriers; wolf packs; and midget submarines. He describes the methods used to hunt and destroy submarines, and relates mistaken identity incidents where submarines of all nations have been sunk in error by forces on their own side. He also examines blockade running; special operations; escape and salvage; and concludes by looking at the future of the world's submarine fleets.