This book, translated by Lev Blumenfeld, contains a synchronic grammar and grammatical dictionaries of Old Church Slavic. The framework is based on a substantially revised version of the classical descriptive methodology. The intent is to improve on the classical monographs by Vaillant, Diels, Lunt in the direction of utmost completeness, explicitness, and deliberate consistency between the grammatical structure, the corpus of texts (limited to the seven oldest OCS manuscripts), and the dictionaries. The grammar is intended as a set of rules that provide a complete characterization of any OCS wordform. Peculiarities in the language of each source are described as systematic departures from canonical OCS, a conventional constructed variety primarily described by the grammar. The book is addressed to linguists working in Slavic studies, as well as to specialists in the general theory of grammar, especially phonologists and morphologists. Data from the dictionaries are freely available as a searchable database online here.