The eucharistic crisis of the eleventh century posed the greatest challenge to the Church's understanding of the Real Presence of Christ in the Blessed Sacrament until the Reformation. The eucharistic symbolism of ""Berengarius of Tours"", which was at the heart of the controversy, was challenged first by Lanfranc of Canterbury and then by his student Guitmund of Aversa. Both authors countered with a vigorous defense of the Church's traditional belief that the body of Christ in the Eucharist is the same body that was born of the Virgin Mary, now risen and in glory. In this first English translation of Lanfranc's ""De corpore et sanguine Domini adversus Berengarium"", the reader learns firsthand both the history of the crisis and the doctrinal issues in question. Lesser known than Lanfranc's work, but of greater doctrinal significance, is Guitmund's ""De corporis et sanguinis Christi veritate in eucharistia"". In Guitmund's work, one finds a treatment of the doctrinal issues involved that is not only more systematic than that of Lanfranc, but far more speculative in character, and one that presents a fascinating vision of the Eucharist as a continuation of Christ's Easter appearances. Such a vision is one that the translator calls the species domini or an 'appearance of the Lord'; for it interprets the doctrine of the Real Presence in a way that identifies it as physical contact with the celestial Christ, of the same genre as the post-Resurrection appearances to his followers. The translations of both Lanfranc's and Guitmund's works, along with extensive commentary and notes, make this volume of the ""Mediaeval Continuation of the Fathers of the Church"" series an important study in the history of the development of eucharistic theology.