This thesis addresses a significant, though largely unexplored, part of the Marian Counter-Reformation. Queen Mary and her ministers expected the University of Oxford's contribution to the success of their plans for the English Church to be decisive. From her letter to the University in August 1553, only weeks after her accession, in which she announced her intention of laying the foundations of her ecclesiastical policy in Oxford, the academy underwent a transformation. After decades of trauma which had left the University poor, empty and (literally, in some parts) crumbling, Mary's reign gave the University a purpose, something which had been difficult to discern since the Dissolution of the Monasteries had deprived it of a large proportion of its students and lecturers. Mary and, after November 1554, Reginald Cardinal Pole undertook an extensive programme designed to reform and restore the University, a programme which was willingly and tirelessly taken up by those sympathetic to it in the University. This had its theological, ecclesiastical, liturgical and architectural elements, each of which will be considered in this thesis. Its central claim is not just that the existing picture of Mary Tudor's Church is incomplete without the inclusion within it of the restoration of Catholicism in Oxford, but that it is in Oxford, and perhaps only there, that all the different elements of her religious policy can be seen for what they are: a consistent whole, conceived and executed with one purpose: the reintegration of the English Church into the universal Catholic body.