Integrating the brilliant biography of Bruno, Archbishop of Cologne (953-65) and brother of Emperor Otto I, by the otherwise obscure monk Ruotger, with the intellectual culture of Cologne Cathedral, this is a study of actual politics in conjunction with Ottonian ruler ethic. Our knowledge of Cologne intellectual activity in the period, apart from Ruotger, must be pieced together mainly from marginal annotations and glosses in surviving Cologne manuscripts, showing how and with what concerns some of the most important books of the Latin West were read in Bruno's and Ruotger's Cologne. These include Pope Gregory the Great's Letters, Prudentius's Psychomachia, Boethius's Arithmetic, and Martianus Capella's Marriage of Philology and Mercury. The writing in the margins of the manuscripts, besides enlarging our picture of thinking in Cologne in itself, can be drawn into comparison with the outlook of Ruotger. Exploring how distinctive Cologne was, compared with other centres, Henry Mayr-Harting brings out an unexpectedly strong thread of Platonism in the tenth-century intellect. The book includes a critical edition of probably the earliest surviving, and hitherto unpublished, set of glosses to Boethius's Arithmetic, with an extensive study of their content.