The wonderful and brave story of ships and men here presented needs but the briefest introduction. The deeds will forever remain one of the most glorious chapters in the chronicles of the sea. No excuse is offered for adding another volume to the literature of the war, for the subject is deserving of greater attention than has hitherto been possible. Lord Jellicoe once remarked that he did not think English people realized the wonderful work which these mystery ships had done in the war, and that in these vessels there had been displayed a spirit of endurance, discipline, and courage the like of which the world had never before seen. To few naval historians, I believe, has it ever been permitted to enjoy such complete opportunities for acquiring authentic information as is here presented. Unquestionably the greatest sphere of Q-ship operations was off the south-west coast of Ireland, owing to the fact that the enemy submarines from the summer of 1915 to 1918 concentrated their attacks, with certain intervals, on the shipping in the western approaches to the British Isles. It was my good fortune during most of this period to be at sea patrolling off that part of Ireland. These Q-ships were therefore familiar in their various disguises at sea or in harbour at Berehaven and Queenstown during their well-earned rest. Throughout this time I kept a diary, and noted down much that would otherwise have been forgotten. Many of the Q-ship officers were my personal friends, and I have enjoyed the hospitality of their ships. Valuable data, too, were obtained from officers of merchant ships who witnessed Q-ships engaging submarines. A considerable number of authentic manuscripts has been examined. By the courtesy of commanding officers I have been lent documents of priceless historical value, such as copies of official reports and private diaries, plans, sketches, photographs, and so on.