Main topic of the doctoral thesis is work and influence of the Archbishop of Zagreb, later Cardinal, Franjo Šeper studied in the context of Church – State relationship in Yugoslavia. Despite the fact that Yugoslavia was governed by the Communist Party Catholic Church has played important role in political, social, economical and cultural life in Croatian society. Being one of the most prominent members of the Catholic Church Franjo Šeper represented important figure in the contemporary society. He was Ordinary of the largest ecclesiastical unit in Yugoslavia. In the same time he took his position in extremely difficult times for Catholic Church. There is no doubt that his successful management of the Metropolitan Archdiocese of Zagreb in those difficult and demanding times along with his activities for the welfare of the Church in general, and especially his engagement on Council and afterwards, resulted in his appointment to the position of Prefect of Sacred Congregation for the Doctrine of the Faith in 1968. Having in mind responsibilities of the Prefect of the Sacred Congregation for the Doctrine of Faith who is responsible for implementation of Council decision one can state that with that appointment Franjo Šeper de facto became Papal “right hand”, and one of the most prominent figures in the contemporary Church. Thesis is divided in seven parts which chronologically follow Šeper’s life.