Epistola de ignoto cantu by Guido of Arezzo is one of the most renown medieval texts on music. Here Guido introduced solmization syllables ut re mi fa sol la which are still used by musicians. Strangely enough, until recently the original had to be studied after the unreliable and fragmentary Martin Gerbert edition of the 18th century (“Scriptores ecclesiastici de musica sacra potissimum”). This new Russian translation of the Epistola is based on critical editions of Dolores Pesce (1999) and Angelo Rusconi (2008). Besides the revised original text (with some crucial enhancements) these editions contain a dozen of music examples which previously have not been taken into account by scholars — their transcriptions are included in the current translation. Besides, it includes extensive commentary written with intention to clarify the original loci obscuri. Three annotated color facsimiles from Guidonian manuscripts are attached in the Appendix. The translation is preceded by the Introduction where the author selects some typically Guidonian terms found in Epistola (with emphasis on polysemantic examples) and also outlines his approach to rendering of complicated music terminology to the Russian language. The Introduction also includes a brief survey of the most important topics of Guido’s theory, with an attempt to separate its stereotyped transmission from genuine contribution of the Italian musician.