It is just fifty years since the inception of the liturgical movement associated with the great Benedictine abbey of Maria Laach in the Rhineland. What has come to be known as the theologia lacensis, the theology of Laach, is only the doctrinal side of a multiple appeal to the Catholic world to foster its truest insights, an appeal which goes out just as much in the way the monks worship in the ancient basilica and in the works of religious art that come from the ateliers. But whatever the place and importance of other aspects of this manifold undertaking, the theological ideas which derive from Maria Laach have proved by far its most decisive influence on the general life of the Church. The most characteristic of these ideas are those of Dom Odo Casel. Born at Coblenz in 1886, he entered the monastery in 1905 and was soon caught up in the liturgical and patristic research which was to form his life's work.