This book traces the presence of the Holy Spirit in worship and in the Scriptures from the Old Testament, where the imagery used is that of the "ruah" or "breath" of God, to the teachings of our Lord and their development in St. John's Gospel and the descent of the Holy Spirit upon the Apostles at Pentecost. The author traces the emergence of Trinitarian theology in the early centuries of the Church and its clarification in the writings of St. Augustine and St. Thomas Aquinas. The question of the "procession" of the Holy Spirit which still divides the Eastern and Western Churches is discussed in detail, together with the meaning of the sending or mission of the Holy Spirit. The titles of the Holy Spirit in liturgy and tradition are also described and the Holy Spirit's mission is revealed as the completion of the work of Christ and the means by which the mystery of the Word Incarnate remains actual and always efficacious.