When in 1592 the newly elected ponti? Clement VIII paid a visitation to Romeߣs principal church of San Giovanni in Laterano, he renovated the Basilicaߣs transept and installed at its southern end a separate altar where the Holy Sacrament of the Eucharist would be preserved. Through this architectural intervention, which transferred the Canonߣs Sacristy outside the church interior, it was clear that the navata clementina would need a ?tting iconographic programme to embellish the new architecture. This chapter argues that these developments must be seen in the context of contemporary debates about the reform of the Roman rite.It also identifies the newly elected Cardinal and Oratorian from Sora, Cesare Baronio, the man who composed the ?rst ocial Catholic historiography of the Roman Church, the Annales Ecclesiastici, as playing a hitherto unidentified but crucial role in the design of the new architecture and its iconographic programme, a programme encompassing eight large scenes taken from the life of Constantine the Great.