ENThis article focuses on the beginnings and the organizational structure of the Diocese of Vilnius as well as on the most important diocesan administrative institutions: the episcopal see and the cathedral chapter.The paper discusses the emergence of the parish structure and the development of monastic communities in the diocese. Finally, it provides an overview of the endowments bestowed on the cathedral by members of nobility and presents the history of the cult of most popular saints. The Diocese of Vilnius was erected in 1387 when Lithuania converted to Christianity. Vilnius Diocese as well as the Diocese of Luck (established in 1400) and the Diocese of Samogitia (established in 1417) covered the entire territory of the Grand Duchy of Lithuania. It was the largest, among the dioceses of Poland and Lithuania, as it covered about two-thirds of the Grand Duchy. With a vast territory amounting to 226 000 square kilometers, it was the largest diocese in the country and one of the largest in Europe. Spiritual and ecclesiastical supervision of the diocese was carried out by the bishops of the diocese who were members of the Lithuanian Council of Lords between the fifteenth and sixteenth centuries. They would later become senators and get actively involved in the political life of the country. The first royal endowment was bestowed on Vilnius episcopal see by King Ladislas Jagiełło. Subsequent foundations were made by royal as well as private donors. Bishops collected significant revenue. In mid-sixteenth century the endowment of the Bishop of Vilnius was 3.5 times higher than the total income of the cathedral chapter. Bishops were elected by the chapter and nominated by the pope. In the fifteenth and sixteenth centuries a lengthy formal procedure had to be followed before a bishop would be instituted in his office.The royal nomination of the candidate, elections in the chapter, and papal nomination could last about a year. Vilnius episcopal see was the highest and most prominent ecclesiastical office in Lithuania as far as the episcopal endowments and the role the Bishop of Vilnius played in the Senate. The Dioceses of Luck, Samogitia and Kiev were lower in rank. Therefore Luck and especially Samogitian ordinaries would make attempts to advance to the position of the Bishop of Vilnius. Vilnius Cathedral Chapter received formal papal approval in 1388 and was founded by Kingjagiello on 15 May 1390.The chapter consisted of she prelates and twelve canons - this arrangement had been introduced around 1524 and lasted until the end of the eighteenth century. The chapter shared in diocesan administrative duties with the bishop and it would often delegate a candidate for the ordinary from among its members. It was also to assist the bishop in the government of his diocese. As in the cases of all other cathedral chapters, in Vilnius, too, the prelacies were appointed different duties. The duty of the dean was to convoke the chapter and to supervise the cathedral clergy: vicars, prelates, presbyters. The archdeacon acted as the bishops representative with the duty of supervising the preparation of the episcopal visitations in the diocese.The church of St Stanislas and St Ladislas situated in the castle of Vilnius was first mentioned in 1387. The first cathedral was a stone building. After the first fire in 1419 it was rebuilt as a three-nave Gothic style building with vaulted ceiling. The church underwent major reconstruction after the 1530 fire. The present building was built by Wawrzyniec Gucewicz in Neo-Classical style at the end of the eighteenth century. The cathedral was raised to the rank of Basilica in 1889, and when the Archdiocese of Vilnius was established in 1925, it became Archcathedral. The cathedral has been closed down during the long period of the Soviet occupation and between 1956-1988 it housed a picture gallery. The Chapel of Saint Casimir is undoubtedly the glory of the cathedral. It is a remarkable site of devotion to Saint Casimir who died in 1484 and was a son of Casimir Jagiellon, King of Poland and Grand Duke of Lithuania. King Sigismund III Vasa and King Ladislas IV Vasa commissioned artists to build the Chapel in 1623-1636. The relics of the pious prince were then solemnly transferred to the Chapel where they remain until the present day with some interruptions. [...].