The Bibliotheca (Greek: Βιβλιοθήκη) or Myriobiblos (Μυριόβιβλος, "Ten Thousand Books") was a ninth-century work of Byzantine Patriarch of Constantinople Photius, dedicated to his brother and composed of 279 reviews of books which he had read. It was not meant to be used as a reference work, but was widely used as such in the 9th century, and is generally seen as the first Byzantine work that could be called an encyclopedia. The works he notes are mainly Christian and pagan authors from the 5th century BC to his own time in the 9th century AD. Almost half the books mentioned no longer survive. Reynolds and Wilson call it "a fascinating production, in which Photius shows himself the inventor of the book-review". and say its "280 sections... vary in length from a single sentence to several pages".
Some older scholarship had speculated that Bibliotheca might have been composed in Baghdad at the time of Photius' embassy to the Abbasid court, since many of the mentioned works are rarely cited during the period before Photius, i.e. the so-called Byzantine "Dark Ages," (c. 630-c.800), and since and it was known that the Abbasids were interested in translating Greek science and philosophy. However, modern specialists of the period, such as Paul Lemerle, have pointed out that that this cannot be the case, since Photius himself clearly states in his preface and postscript to the Bibliotheca that after he was chosen to take part in the embassy, he sent his brother a summary of the works he had read previously "since the time that I learned how to understand and evaluate literature," i.e. from his youth. A further difficulty with supposing that Bibliotheca was composed during rather than before the embassy, besides Photius' own explicit statement, is that the majority of the works in Bibliotheca are of Christian patristic theology, and most of the secular works are histories, grammars, and works of literature, particularly rhetoric, rather than works of philosophy or science, and the Abbasids showed no interest in having Greek history or Greek high literature like rhetoric translated, nor were they interested in translating Greek Christian works. Their interest in Greek texts was confined almost exclusively to science, philosophy and medicine. In fact, "there is almost no overlap (other some Galen, Dioscurides, and Anatolius) between the inventory of secular works in Photius's Bibliotheca and those works that were translated into Arabic"  in the Abbasid period. This is mainly due to the Abbasids' lack of interest in the sort of Greek historical, literary, and theological texts that Photius tended to favor in Bibliotheca.
- Reynolds, L. D and N.G. Wilson (1991). Scribes and Scholars: A Guide to the Transmission of Greek and Latin Literature (3rd ed.). Oxford: Clarendon Press. p. 321. ISBN 0-19-872145-5.
- Paul Lemerle, Byzantine Humanism: The First Phase: Notes and Remarks on Education and Culture in Byzantium from its Origins to the 10th century. Canberra: Australian Association for Byzantine Studies, 1986, 81-84.
- Jokisch, Islamic Imperial Law (2007), pp. 364-386)
- As Lemerle says "it is clear that this was written, and therefore that the collection has been composed, before the completion of the embassy. Lemerle, Byzantine Humanism: The First Phase, 40.
- As Paul Lemerle puts it, "while the renaissance in Hellenism in Byzantium extended progressively and more or less rapidly to all spheres of ancient literature including poetry, theatre, rhetoric, history, etc., Islam took an interest only in the sciences on the one hand, and philosophy on the other, Byzantine Humanism: The First Phase, 26-27
- Dmitri Gutas, Greek thought, Arab Culture, 1998, 186.