The manuscript called the Book of Deer, with the notes on grants of land to the monastery of Deer written into it in the Gaelic spoken in Buchan towards the middle of the twelfth century, has been known ever since it was ‘re-discovered’ in 1860, as is described below. The Gaelic notes have been edited, with or without facsimiles, translated, and discussed a number of times, so that one might suppose that any further treatment was unnecessary. But no completely satisfactory edition, or translation, exists as yet, as appears from the comments in this Preface, and some of the conclusions which have been drawn by previous authors about the social system and the principles of land-holding and land-granting, taxation, etc., in the north-east of what was still Celtic Scotland in the twelfth century, and about the date, genuineness, and language of these‘charters’, need re-assessment. These documents are, after all, unique, and of unique interest to the study of the language and history of mediaeval Celtic Scotland, Scotland before the process of Normanisation which began there early in the twelfth century under Alexander I and David I had had more than the most superficial effect in the north. The language is the Gaelic used by the upper classes of northern Scotland of that period, and it is instructive to see that it is virtually indistinguishable from contemporary Irish, as will be shown in detail in this book.