During the Second World War, German U-boats played a central role in offensive operations across the world's oceans. In the Battle of the Atlantic, U-boats were pitched against Allied merchant ships to prevent vital supplies being brought to Britain – Hitler’s undersea fleet brought Britain to the brink of capitulation. This is the first comprehensive guide in English to the bases that lay behind this deadly operation. Hitler ordered these bases to be constructed at strategically crucial sites, where the U-boats were berthed and repaired, and from where they sailed forth on their war patrols. Many of these monolithic structures still exist today in all their menacing glory, despite suffering repeated attempts by Allied bombers to destroy them. This book provides a concise historical background to the rise of the Nazi U-boat fleet, and the part it played in the Second World War. The author examines in detail how and why each of the bases in France, Germany and Norway were designed and built, and how they were defended against attack, while listing which boats were based where and when. A comprehensive gazetteer reveals what is left to see today of these massive structures at each of the sites.