This paper presents the author’s personal views on the problems of critical editions of Byzantine and Old Slavonic services. The most significant problem of all relates to the nature and structure of these works. Their form is subject to constant change, because every component of the Orthodox service can easily be changed, removed or added. Moreover, the Slavonic texts present language-related problems which are largely absent from the Byzantine texts: the less imposition of the classical linguistic norm, which results in too many variations and the infiltration of elements from the Mediaeval idioms and, later, from the languages of the Slavic peoples into which the works were rendered; the well-documented differences in rhythm and stress introduced during the process of translating Byzantine texts into Old Slavonic. All the above formulated renders the classic concept of a critical edition inapplicable in the case of Mediaeval Orthodox services. The critical edition of these texts can seek only to restore the ancient core of a service and the older texts that are included in all its known variations. In the same time the edition’s aim should be to present a text that is readable and comprehensible in contemporary terms, and which is readily accessible to a readership beyond specialists in its specific period and subject. Less serious difficulties, related to the critical editions of the services, are: the multitude of copies of even a single service, coupled with the fact that many of these copies are to be found in collections which remain inaccessible, the lack of an all-embracing study of the history, the structure and evolution of the canon as a literary genre, and as far as the Old Slavonic texts are concerned, the lack of certain basic instrumenta studiorum.