The history of Old Church Slavic (also known as Old Church Slavonic or Old Bulgarian) and its descendants begins in 862 CE, when the Byzantine missionaries and brothers Cyril (formerly Constantine) and Methodius were sent to Great Moravia to support Slavs in the area who had converted to Christianity. To bolster local church growth, the brothers translated religious texts such as the Bible into Old Church Slavic, a South Slavic language based on the Salonica dialects which Cyril and Methodius had encountered in their hometown of Thessalonica (modern-day Greece). It is important to note that Old Church Slavic as applied by Cyril and Methodius was a literary language not intended for everyday communication. That is, it was a language never organically spoken but always used in writing. Although it remains a matter of debate whether or not the Slavs had devised their own writing system by the ninth century, Cyril created the Glagolitic alphabet based on Greek minuscule to produce translations into Old Church Slavic. Later in the 11th century, scholars in the Preslav Literary School of the First Bulgarian Empire created the Cyrillic alphabet based on Greek uncial, which gradually overtook Glagolitic as the alphabet of choice for Old Church Slavic. Although it has undergone multiple reforms throughout its history, Cyrillic is still used as the alphabet for several Slavic languages, including Ukrainian, Russian and Bulgarian.

Over time, the liturgical language of Eastern Orthodoxy in the Slavic world began to increasingly reflect distinct regional dialects. This led to the emergence of Church Slavic, a later stage of Old Church Slavic that consists of multiple recensions (regional variants). Recensions of Church Slavic developed in several present-day countries of Eastern Europe, including Bulgaria, Croatia, the Czech Republic, North Macedonia, Romania, Russia, Serbia and Ukraine. Influences of Church Slavic can be found in the literary forms of many of the languages from these countries. An example is the phenomenon of diglossia in the case of Russian, when both vernacular Russian and the Russian recension of Church Slavic were both used by speakers depending on the topic and context (whether secular or religious). As Church Slavic persists as the language of liturgy for the Eastern Orthodox Church, religious materials are still published in the language. This makes Church Slavic an excellent resource for studying Eastern Orthodoxy and its history.

This guide is intended to be a starting point for research on Old Church Slavic and Church Slavic using sources found at the Library of Congress. The Library of Congress has a vast collection of primary and secondary sources both written in and about Old Church Slavic and Church Slavic, including Bibles, Gospels and other original texts dating from as early as the 15th century.

Tip: When searching the Library of Congress Online Catalog, it is important to keep in mind that Old Church Slavic can also be referred to as Old Bulgarian, Old Slavonic or Old Church Slavonic (OCS) in bibliographic records. Furthermore, Church Slavic is used synonymously in the catalog with New Church Slavonic. Finally, titles in Cyrillic must be transliterated into Latin letters before entering them into the online catalog's search function. Library of Congress Romanization standards for Slavic languages, including Church Slavic, can be found here