Old Church Slavonic Institute in Zagreb, founded in 1952, is devoted to the research of the Croatian language and literature from the time when the Croats, first in their principality, later in their kingdom, participated with other European peoples in establishing the historical, political, spiritual, and civilization foundations of modern Europe. Therefore, the scholars of the Old Church Slavonic Institute study the medieval chapter of the Croatian language and literary culture. These studies, which started in the 18th century, developed more profoundly at the end of the 19th and in the first half of the 20th c., and fully flourished in the second half of the 20th century mainly due to the work at this Institute. The Croatian medieval language and literary culture has a significant Latin part as Latin is the first literary language in Croatia, while the Croatian vernacular literature is written in three scripts: Glagolitic, Latin and Croatian Cyrillic. Out of this rich heritage the Old Church Slavonic Institute investigates the Croatian Glagolitism. The Croatian Glagolitism has its roots in the common Slavic cultural heritage, based on the 9th century missionary and cultural work of the Byzantine teachers from Thessalonica, Constantine-Cyrill and Methodius. In later centuries it underwent numerous developmental phases of European medieval Christian spirituality and creativity, until the quality and richness of its texts reached the highest levels of the Latin Western Christian literary civilization in the late Middle Ages. Thus Croatian Glagolitism joined the Croatian Latin literature which has always kept the pace with the contemporary Europe. Numerous works of the Old Church Slavonic Institute devoted to the Croatian Glagolitism during the last decades have proved such a development of the Croatian medieval culture, therefore many medieval topics and entire literary corpora are studied in collaboration with medievalists of Western European universities and institutions. The continuation of research, a more profound and wider study of the Croatian Church Slavonic language, Glagolitic paleography and medieval literature has a significant place in the long-term development of the Republic of Croatia. That culture is one of the first and most important guardians of Croatian national, historical and cultural identity. Together with the Latin culture, it is a witness of Croatia’s centuries long share in the cultural development of Europe up to the present days.