The hotly debated ecumenical and theological issue of whether the Holy Spirit proceeds from the father alone or from the Son as well as the Father (ex patre filioque) is analysed in light of ecclesiastical history and the history of Christian doctrine. Starting from the varied reception of the Niceno-Constantinopolitan creed in the Latin-speaking area, the author inquires into the origins of the filioque controversy between the eighth and twelfth centuries. The pneumatological and confessional differences and the basic political and ecclesiastical framework involved are described. The dispute over the filioque clause proves to be a key to a theological divergence between the Greek East and the Latin West, a movement whose origins lay in the fundamental theological decisions about the Trinity in the fourth century. The filioque issue thus cannot be "solved" by deleting a word but only by ensuring dialogue between two irreducible manifestations of the Trinitarian dogma.