In the postconciliar period, among the scholars who articulated the urgent need to restore pneumatology to its rightful place in the theology of Catholic Church was Cardinal Yves Congar. This book centers on one aspect of this theme in which his influence is particularly evident, namely, the constitutive role of the Holy Spirit in effecting the sacraments of the Church. Through a careful analysis of the texts of Congar which explicitly treat the sacramental epiclesis, the author claims that the solemn invocation of the Holy Spirit is as constitutive of the sacraments of the Church as is the formal recitation of the words of Jesus Christ.