Book Nine of the series "Cyril and Methodius Studies" contains the first monographic study of the work of the prominent 10th-century Old Bulgarian writer Petar Chernorizets. Special attention is paid to determining the scope of his work, the dissemination of his works in Orthodox Slavic countries, and a detailed study of the writer's language. For the first time, the book publishes all of Petar Chernorizets' works after a comprehensive analysis of more than 200 Bulgarian, Serbian, and Russian manuscripts from the 12th to the 18th centuries collected by the author. The publication is accompanied by the first translation of the works into modern Bulgarian.