The article states that the four ancient Greek lives (A, B, C, D) contain extensive information about the life and activity of St. Theodore of Studios (759-826). The oldest one is Vita S. Theodori Studitae B which was published in 1853 by A. Mai. The document was written in c. 868 and its author was probably a monk Michael or another ascetic who lived in the Studios Monastery in Constantinople; Vita A, which is an extended version of Vita B, was published in 1696. His author was Theodore Daphnopates, who lived in the first half of the 10th century. Vita C (Vita S. Theodori Studitae in codice Mosquensi musei Rumianzoviani No 520) was published in 1915, but it dates back to the 12th century and it is similar in its content to Vita S. Theodori Studitae A and B. Vita D (Codex Monacensis No 467) was not yet published.