As was the case in World War II, one of the greatest threats to Britain during World War I was the German U-Boat menace. Gordon Williamson traces the development of the U-Boat from the Brandtaucher, designed by Wilhelm Bauer, the father of the German submarine arm, in 1850, through to the commissioning of Germany's first U-Boat to go into service, the U-1, in 1906. All of the main types of World War I U-Boat are then covered. The operational history of the U-Boat service is also covered in some depth. Included are details of the campaigns in the Atlantic and Mediterranean, along with the slow build-up of anti-submarine measures by the allies.