This dissertation is an attempt to identify the operative pneumatology of the Second Vatican Council. The study will probe not only the final documents but also the event itself by looking for the pneumatology that guides the changes in the conciliar processes and procedures and in the speeches of the participants. There are two warrants for grounding the study in pneumatology. The first is that the activity of the Holy Spirit is intimately and inextricably associated with the life of the Church. The second is that the council is often called a 'Pentecostal event.' Gustave Thils offers a method for such an exploration of the Council's underlying pneumatology. He suggests that we interpret the Council as a event by attending to the 'trajectory' in the various theologies at the Council. This trajectory indicates which ideas acquired increasing importance and which ideas and concepts consistently lost in importance. By tracing the various conciliar notions of God's presence and activity in the Church and in the world, as manifested in the speeches, processes and texts, this dissertation seeks to describe and clarify this 'trajectory.' Such a clarification may then be applied when the final documents themselves are interpreted. Using pneumatology as a basis of measurement, we shall study the varying understandings of the Holy Spirit of the Council members, their theologian advisors, the powerful Vatican Cardinals and, of course, the two Popes of the Council, John XXIII and Paul VI. The study will compare the pneumatology developed during the Council and the theology that was available at the time. Placing the dominant pneumatology that emerged from the Council in dialogue with the state of pneumatology in contemporary Catholic thought will provide a corrective for pneumatological discussions that fail to account for the intimate relationship of the theology of the Holy Spirit with the life of the Church. In addition, the theological description of the pneumatology of the Council may be used in the further interpretation of the Council's texts.