One of the characteristics of the Enlightenment was the willingness to push beyond personal intellectual boundaries: to try to understand adjacent disciplines, to master them and take them forward. The autodidact could break through, enabling the individual to become eminent in an area of study. The word polymath appeared in the early seventeenth century to describe deep learning across several different subjects, endowing this unfamiliar ability with an air of admiration and perhaps a little bemusement also. This is not to claim that William Roy was a polymath, but he was someone who honestly engaged with the diverse areas of his ignorance that he regretted, and then went about addressing them so successfully that he could become a practitioner. He had several books in his library that were explicitly introductions to subjects, and their presence demonstrates his willingness to learn throughout his life. Now, in contrast, we live in an age of specialism; most of us have either lost that desire for broad and continual self-improvement, or we are wary of exercising it, so there are problems that have to be faced in the writing of a biography of a man like William: it is a cross-disciplinary subject, and this means that little knowledge can be assumed on the part of the reader who may come to it from one direction or another. At the same time, specialists may feel that their own sphere of interest has been under-represented, or not treated in sufficient detail, or it has been described only at a level that is simplistic so that readers from other specialisms may readily understand it. In trying to achieve some balance, tolerance of the needs of others may be required. Thus the references in the notes to each chapter often act as pointers to deeper or wider consideration of the relevant topic, especially for those who are tempted to explore beyond their own particular discipline. If readers wish to do this, in emulation of William’s own willingness to cross boundaries, it will be a fitting by-product. In addition, a short glossary has been included in Appendix 3 so that the terms that may be perfectly familiar to an archaeologist, say, may be explained to a geodesist, and vice versa.