During the Second World War the Germans developed a specially adapted U-boat oil tanker with two aims. First, by refuelling the attack U-boat fleet their range of operations and duration of patrol could be significantly increased. Secondly, these underwater tankers were far more likely to avoid detection than surface support ships. The submarine tankers, affectionately known as Milk Cows, were regarded by both the Germans and the Allies as the most important element of the U-boat fleet. Allied forces had orders to attack the tankers first whenever a choice was presented. Until late 1942 the German Milk Cows operated with great success and few losses. But from 1943 onwards the German rendezvous ciphers were repeatedly broken by the Allies and losses mounted rapidly. The Milk Cows were highly vulnerable during the lengthy refuelling procedure as they lay stationary on the surface, hatches open. By the end of the war virtually every tanker had been sunk with severe loss of life. The story of this critical campaign has been thoroughly researched by the author and is told against the background of changing U-boat fortunes.