The fascinating subject of superconductivity was opened over a century ago by Onnes [1], but until quite recently it was strictly a low-temperature phenomenon. The discovery of the cuprate superconductors [2] in a family of transition metal oxides, with transition temperatures up to Tc ~ 100K, has generated tremendous excitement for two main reasons. First, from a practical point of view, these compounds open a new temperature realm for superconducting devices which may have interesting commercial applications, and these potential benefits have attracted extraordinary attention from the whole scientific community. The second reason, relevant to those in a more abstract field, is the interest in the microscopic mechanism driving superconductivity.