To the old saying that if you want to learn something about a subject, teach a course on it, I would add the much more arduous option of writing a book about it. The upside, of course, is that you may gain a perspective on a lo- time favorite subject which can be achieved in no other way. The fact is that the opportunity to write such a book as this is, indeed, a rare opportunity. This is especially so for someone who, by the calendar, is well into retirement years. While high T itself is the opportunity of a lifetime for those of us who c experienced it, and revisiting the old milestones in writing this monograph was a genuine nostalgia trip, it is also important to re?ect on the purpose of such a book as this. Now that the great majority of experimental high-T c studies are ?nished, who could be interested in such a book? To give some 2 1 kind of answer to this, let me pinpoint the 2006 M S Conference in Dresden.

The NMR probe has yielded a vast array of data for the high-T _c materials, corresponding to different compounds, ionic sites, and nuclear species, as well as to a wide variety of experimental conditions. Over the twenty years, since the discovery of superconducting cuprates, ongoing analysis and discussion of cuprate NMR data have resulted in a wealth of important insights into the physics of these exotic systems. The aims of this monograph are threefold. First, it reviews NMR methodology as it has been applied to the cuprate studies. This is addressed to NMR practitioners and to physics laypersons alike. Next, it presents a review of cuprate NMR measurements and the wide variety of phenomena which they represent. The third phase is to recount the theoretical model calculations and other proposals which have been put forward to account for these data. Parts two and three are presented in parallel, as there are many aspects to both topics, each with its own interesting history. There is, even twenty years on, a substantial level of current theoretical development underway on the topic of high-T _c . Thus, there are theoretical controversies to be addressed, notably that between the one and two-component pictures of NMR dynamics, which may not be resolved for some time to come. Further, even though one would have to say that the experimental picture is largely settled for high-T _c NMR studies, theoretical controversies may legitimately call for new, improved or more extensive data. There are also apparent conflicts between NMR and neutron scattering results. These must be addressed and rationalized very carefully. What remains in the end, then, is a snapshot of an ongoing enterprise, which is undoubtedly the grandest program ever undertaken under the rubric of condensed matter NMR studies.