This is a revised and translated edition of Gilbert Dagron's Empereur et prêtre, an acknowledged masterwork by one of the great Byzantine scholars of our time. The figure of the Byzantine emperor, a ruler who sometimes was also designated a priest, has long fascinated the western imagination. This book studies in detail the imperial union of 'two powers', temporal and spiritual, against a wide background of relations between Church and state and religious and political spheres. Presenting much unfamiliar material in complex, brilliant style, it is aimed at all historians concerned with royal and ecclesiastical sources of power.