The French Dominican theologian Yves Congar is recognized by many as the most important Roman Catholic ecclesiologist of the twentieth century. He was the thinker behind some of the major decrees of the Second Vatican council. He was also a leader in the ecumenical movement in Europe throughout most of the century. Despite his importance, there are few books about Congar in English. Congar’s pneumatology, argues Groppe, can enrich various ongoing theological discussions, including reflection as to whether the church should be hierarchical or a democracy, the development of “persons in communion” as a framework for contemporary theological anthropology and ecclesiology, and deliberations about the personhood of the Holy Spirit.