Winston Churchill famously claimed that the submarine war in the Atlantic was the only campaign of the Second World War that really frightened him. If the lifeline to north America had been cut, Britain would never have survived; there could have been no build-up of US and Commonwealth forces, no D-Day landings, and no victory in western Europe. Furthermore, the battle raged from the first day of the war until the final German surrender, making it the longest and arguably hardest-fought campaign of the whole war. The ships, technology and tactics employed by the Allies form the subject of this book. Beginning with the lessons apparently learned from the First World War, the author outlines inter-war developments in technology and training, and describes the later preparations for the second global conflict. When the war came the balance of advantage was to see-saw between U-boats and escorts, with new weapons and sensors introduced at a rapid rate. For the defending navies, the prime requirement was numbers, and the most pressing problem was to improve capability without sacrificing simplicity and speed of construction. The author analyses the resulting designs of sloops, frigates, corvettes and destroyer escorts and attempts to determine their relative effectiveness.