For just over a thousand years the Virgin Mary was central to any attempt to defend or explain Christian Orthodoxy. From the formulations at Ephesos and Chalcedon Mary formed part of the understanding of a God made Flesh and of a picture of redemption which was all-embracing in its promise and tantalizing in its accessibility. This essay shows just how wide and diverse were the medieval ways of thinking about Mary and the ways of exploring the possibilities inherent in the figure of the Mother of God. In liturgy and prayer, in homilies and devotional poetry, in a vast array of material forms Mary was made familiar, above all as mother, as intercessor and companion. Unlike the sacraments, among them the all-important Eucharist, Mary was rarely a subject of discipline or of scrutiny; she entered people’s lives early and seemingly effectively. She stood, however, as a boundary-marker of Christian identity, the quintessential barrier between Christians and Others. Mary did become a subject of discipline to people in the lands of conquest and disease outside Europe.