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. All this information has been further augmented by personal conversation, correspondence, and valuable criticism. I submit, therefore, that with all these sources of information available, and with knowledge of much that has been published from the German side, it is possible to offer a monograph that is at once accurate in detail and correct in perspective. ‘With respect to single-ship actions,’ wrote James in his monumental Naval History a hundred years ago, ‘the official documents of them are also very imperfect. The letters are generally written an hour or so after the termination of the contest, and, of course, before the captain has well recovered from the fatigue and flurry it occasioned. Many captains are far more expert at the sword than at the pen, and would sooner fight an action than write the particulars of one.’ That statement is true to-day of the Q-ships, and it would have been negligent not to have availed oneself now of the calm and considered version of the chief actors in the great mystery-ship drama while they are still alive. Although the time for secrecy has long since passed, nothing has here been included of a confidential nature that can be of assistance to enemies past or potential. In one instance, for political reasons and in the interests of the service, I have made a certain omission. Those concerned will recognize this and understand: the rest will not notice it.