In attempting to determine the medical nature of the various widespread diseases recorded by the Irish chroniclers, my procedure has been to collate the accounts given in the several annals, thus securing the maximum amount of information obtainable about each outbreak. In making these comparisons it was distressing to come across some isolated and incomprehensible statement, clearly a relic of what was once a fuller description but now curtailed through the arbitrary omission of details which the early transcribers did not understand. Where Ireland was involved in a more generalized epidemic, it was sometimes possible to supplement a meagre native account by drawing on subject matter recorded elsewhere. For example, little could be made of the Irish entries of the pestilence of 664, were it not for the story of its course in England which we owe to the Venerable Bede. These assembled data were studied in the light of present medical knowledge, aided materially in many instances by my own experience of the diseases concerned. In such ways it has proved possible to give a firm and I believe a correct opinion on the identity of some of the pestilences. In other instances, no more can be done than to suggest some probability; and if even this is impossible because of insufficient data, such evidence as there may be is presented for consideration, even though the problem is left unsolved.