As a quasi-historical, quasi-legendary figure of consistently great popularity, King Arthur has been subject to an extraordinary amount of reinvention and rewriting: as a Christian hero and war-leader; as an ineffective king and pathetic cuckold; and as a tragic figure of noble but doomed intentions. As we trace Arthur’s evolution and that of principal knights, we will ask what underlies the appeal of this figure whose popularity through the centuries has performed the medieval prophecy that he would be rex quondam et futurus: the once and future king.