In this study, the author focuses on the development of ecclesiastical law and its role within the Duchy of Bohemia, and the Kingdom of Bohemia. The development of ecclesiastical law within the Czech lands during the Middle Ages can be attributed a number of phases. The first phase is the early period of the Duchy of Bohemia. An important document from this period is the Břetislav, or Gniezno, Decrees. These are evidence of an attempt at eradicating the relics of non-Christian customs within Czech society. One limitation on the development of ecclesiastical administration was the fact that during this early period, individual churches were the property of their founders; the Church did not have control of churches or of priests. Major reforms in his diocese were implemented by Bishop Jindřich Zdík of Olomouc (1126–1150). Copies of the important ecclesiastical law collections of Burchard of Worms and Ivo of Chartres were produced in his Olomouc scriptorium. Until the start of the 13th century, the bishop held the status of Chaplain of the Duke. Subsequently, the bishop’s status began to change. His choice of cathedral chapter was recognised. The impulse for further change was provided by Bishop Ondřej II of Prague’s struggle to implement the basic postulates of the Fourth Lateran Council and the papal programme within the Diocese of Prague. The entire thirteenth century and the first half of the 14th century were marked by the implementation of new rules into the life of society. These involved judicial immunity for Church possessions, the law of patronage, and judicial exemption for the clergy. The influence of the Church and ecclesiastical law reached a peak in the 14th century and early 15th century. This is also the period in which the influence and importance of ecclesiastical law reached its zenith in the Czech lands. Part of the Hussites’ programme was the complete rejection of ecclesiastical law. After the Hussite Revolution, there was no restitution of the previous state, and ecclesiastical law became subject to secular law in society’s mind. In the post Hussite period, fundamental church issues and important matters of religion were discussed at provincial assemblies, to which both legal churches – the Catholic and Utraquist Churches – were now subject. Only the day-to-day management of the Church was left within the competency of ecclesiastical offices.