Ever since the discovery of the high temperature superconductors, one of the most intensively debated questions has been: why are their critical temperatures so much higher than those of the earlier superconductors? In principle, an answer might allow us to predict new superconductors. Understanding the origin of high temperature superconductivity presents a formidable challenge. Some years ago Sir Nevill Mott, questioning if there might be a generally acceptable explanation, concluded that, “There are probably as many theories as theorists”. Today the situation has improved but a definitive theory is yet to emerge.