From 5 October 869 to 28 February 870, as many as 103 bishops, among them the legates of Pope Nicholas i, gathered at Constantinople to confirm the deposition of the patriarch Photius. What had begun in 858 as a strictly Byzantine dispute with the removal of Photius' predecessor, had quickly expanded to embrace such universally intractable problems as Roman primacy, the filioque and the Bulgarian mission. Though the Council of 869/70 resolved the Photian schism, Photius returned to office within a decade, and presided over a second and larger council, also at Constantinople, to confirm his restoration. These rank among the most momentous events of ninth-century church history, with implications for the division between Eastern and Western Christianity that persist to this day. Beyond some excerpts of the original Greek, the acta of the Council of 869/70 survive only in the Latin translation of Anastasius Bibliothecarius, who was present at the proceedings and made his own copy of the documents. Fortunately, Anastasius did not return to Rome with papal legates, who encountered pirates on their westward voyage and lost their sole authorised copy of the conciliar decrees as a result. When Anastasius translated his Greek copy into Latin at the request of Pope Hadrian ii, he thus became responsible for the only complete witness to the conciliar proceedings against Photius. In 1967 Claudio Leonardi discovered that Anastasius' personal draft of the Latin translation survives as Vatican, Bibliotheca Apostolica, Vat. lat. 4965 (siglum: A), and that the entire extant manuscript tradition depends in some way upon this single codex. Despite this crucial discovery, a modern edition has proved to be a monumental undertaking, and the fruits of Leonardi's labours appear here only posthumously. By the time of his death in 2010, Leonardi had completed nearly all the editorial work, leaving only the introduction, some supplements to the apparatus criticus, and the indices for Antonio Placanica to complete. The result is an edition for the ages. The edited text reflects A, as revised by Anastasius; the readings of A before correction, together with the variants of the broader manuscript tradition (all secondary to A) are reported in the apparatus criticus, while an appendix provides select variants from the indirect canonical tradition (Anselm of Lucca through Gratian). An extensive introduction, together with copious indices, enhances usability tremendously; all are in eminently readable Latin. With this secure textual basis, we can await a new era of scholarship on the Photian schism and the tension between Eastern and Western Christian traditions at the end of the ninth century.